diff --git a/src/background/configMigration014.ts b/src/background/configMigration014.ts
index 62f371a327..df508cddef 100644
--- a/src/background/configMigration014.ts
+++ b/src/background/configMigration014.ts
@@ -25,7 +25,7 @@ export default function ({
     if (fs.existsSync(configPath)) {
       const config = JSON.parse(fs.readFileSync(configPath, "utf-8"));
 
-      if (config?.__internal__?.migrations?.version === undefined) {
+      if (config?.__internal__?.migrations?.version == undefined) {
         throw new Error("configMigration014: config.json is invalid");
       }
 
diff --git a/src/components/help/LibraryPolicy.vue b/src/components/help/LibraryPolicy.vue
index ca8a4c4b51..404b61c163 100644
--- a/src/components/help/LibraryPolicy.vue
+++ b/src/components/help/LibraryPolicy.vue
@@ -15,11 +15,11 @@
           <template v-if="engineInfos.size > 1">
             <q-separator v-if="engineIndex > 0" spaced />
             <q-item-label header>{{
-              mapUndefinedPipe(engineInfos.get(engineId), (v) => v.name)
+              mapNullablePipe(engineInfos.get(engineId), (v) => v.name)
             }}</q-item-label>
           </template>
           <template
-            v-for="([, characterInfo], characterIndex) in mapUndefinedPipe(
+            v-for="([, characterInfo], characterIndex) in mapNullablePipe(
               engineInfos.get(engineId),
               (v) => v.characterInfos
             )"
@@ -53,10 +53,10 @@
         </div>
         <div class="text-subtitle">
           {{
-            mapUndefinedPipe(
+            mapNullablePipe(
               engineInfos.get(selectedInfo.engine),
               (v) => v.characterInfos,
-              (v) => mapUndefinedPipe(selectedInfo, (i) => v.get(i.character)),
+              (v) => mapNullablePipe(selectedInfo, (i) => v.get(i.character)),
               (v) => v.metas.speakerName
             )
           }}
@@ -73,7 +73,7 @@ import { computed, ref } from "vue";
 import { useStore } from "@/store";
 import { useMarkdownIt } from "@/plugins/markdownItPlugin";
 import { EngineId, SpeakerId } from "@/type/preload";
-import { mapUndefinedPipe } from "@/helpers/map";
+import { mapNullablePipe } from "@/helpers/map";
 
 type DetailKey = { engine: EngineId; character: SpeakerId };
 
diff --git a/src/helpers/SelectionHelperForQInput.ts b/src/helpers/SelectionHelperForQInput.ts
index 18fb20dee6..c1b28edd66 100644
--- a/src/helpers/SelectionHelperForQInput.ts
+++ b/src/helpers/SelectionHelperForQInput.ts
@@ -11,7 +11,7 @@ export class SelectionHelperForQInput {
 
   // this.start が number | null なので null も受け付ける
   setCursorPosition(index: number | null) {
-    if (index === null) return;
+    if (index == undefined) return;
 
     this.nativeEl.selectionStart = this.nativeEl.selectionEnd = index;
   }
@@ -50,7 +50,7 @@ export class SelectionHelperForQInput {
   get isEmpty() {
     const start = this.nativeEl.selectionStart;
     const end = this.nativeEl.selectionEnd;
-    return start === null || end === null || start === end;
+    return start == undefined || end == undefined || start === end;
   }
 
   private get nativeEl() {
diff --git a/src/helpers/map.ts b/src/helpers/map.ts
index 3b2d170b87..9770c7773a 100644
--- a/src/helpers/map.ts
+++ b/src/helpers/map.ts
@@ -1,36 +1,36 @@
-export function mapUndefinedPipe<T, U1>(
+export function mapNullablePipe<T, U1>(
   source: T | undefined,
   fn1: (_: NonNullable<T>) => U1 | undefined
 ): U1 | undefined;
-export function mapUndefinedPipe<T, U1, U2>(
+export function mapNullablePipe<T, U1, U2>(
   source: T | undefined,
   fn1: (_: NonNullable<T>) => U1 | undefined,
   fn2: (_: NonNullable<U1>) => U2 | undefined
 ): U2 | undefined;
-export function mapUndefinedPipe<T, U1, U2, U3>(
+export function mapNullablePipe<T, U1, U2, U3>(
   source: T | undefined,
   fn1: (_: NonNullable<T>) => U1 | undefined,
   fn2: (_: NonNullable<U1>) => U2 | undefined,
   fn3: (_: NonNullable<U2>) => U3 | undefined
 ): U3 | undefined;
 /**
- * 一連の関数を実行する。途中でundefinedを返すとその後undefinedを返す。
+ * 一連の関数を実行する。途中でundefinedかnullを返すとその後undefinedを返す。
  */
 // eslint-disable-next-line @typescript-eslint/ban-types, @typescript-eslint/no-explicit-any
-export function mapUndefinedPipe(source: any, ...fn: Function[]) {
+export function mapNullablePipe(source: any, ...fn: Function[]) {
   return fn.reduce((prev, curr) => {
-    if (prev === undefined) {
+    if (prev == undefined) {
       return undefined;
     }
     return curr(prev);
   }, source);
 }
 
-export const undefinedToDefault = <T>(
+export const nullableToDefault = <T>(
   defaultValue: T,
   maybeValue: T | undefined
 ): T => {
-  if (maybeValue === undefined) {
+  if (maybeValue == undefined) {
     return defaultValue;
   }
   return maybeValue;
diff --git a/src/helpers/previewSliderHelper.ts b/src/helpers/previewSliderHelper.ts
index fff7ae0106..bebb497cf6 100644
--- a/src/helpers/previewSliderHelper.ts
+++ b/src/helpers/previewSliderHelper.ts
@@ -112,7 +112,7 @@ export const previewSliderHelper = (props: Props): PreviewSliderHelper => {
     previewValue.value = value;
   };
   const changePreviewValue = async () => {
-    if (previewValue.value === null)
+    if (previewValue.value == null)
       throw new Error("previewValue.value === null");
     if (modelValue.value !== previewValue.value && props.onChange) {
       await props.onChange(previewValue.value);
@@ -128,7 +128,7 @@ export const previewSliderHelper = (props: Props): PreviewSliderHelper => {
   };
   // start awaiting
   const fireChange = () => {
-    if (awaitingChange !== null) awaitingChange.cancel();
+    if (awaitingChange != null) awaitingChange.cancel();
     isAwaiting.value = true;
     awaitingChange = new CancelableFinary(changePreviewValue(), endAwaiting);
   };
@@ -165,7 +165,7 @@ export const previewSliderHelper = (props: Props): PreviewSliderHelper => {
   );
   // This function is called when the q-slider fire onWheel.
   const onWheel = (event: Events["onWheel"]) => {
-    if (disableScroll.value || disable.value || currentValue.value === null)
+    if (disableScroll.value || disable.value || currentValue.value == null)
       return;
     event.preventDefault();
     const deltaY = event.deltaY;
diff --git a/src/infrastructures/EngineConnector.ts b/src/infrastructures/EngineConnector.ts
index 55f4d4e8a7..7ddede8273 100644
--- a/src/infrastructures/EngineConnector.ts
+++ b/src/infrastructures/EngineConnector.ts
@@ -11,7 +11,7 @@ const OpenAPIEngineConnectorFactoryImpl = (): IEngineConnectorFactory => {
   return {
     instance: (host: string) => {
       const cached = instanceMapper[host];
-      if (cached !== undefined) {
+      if (cached != undefined) {
         return cached;
       }
       const api = new DefaultApi(new Configuration({ basePath: host }));
diff --git a/src/store/audioPlayer.ts b/src/store/audioPlayer.ts
index 966a6f1429..7785b2b814 100644
--- a/src/store/audioPlayer.ts
+++ b/src/store/audioPlayer.ts
@@ -23,7 +23,7 @@ export const audioPlayerStoreState: AudioPlayerStoreState = {
 export const audioPlayerStore = createPartialStore<AudioPlayerStoreTypes>({
   ACTIVE_AUDIO_ELEM_CURRENT_TIME: {
     getter: (state) => {
-      return state._activeAudioKey !== undefined
+      return state._activeAudioKey != undefined
         ? getAudioElement().currentTime
         : undefined;
     },
@@ -61,7 +61,7 @@ export const audioPlayerStore = createPartialStore<AudioPlayerStoreTypes>({
     ) {
       const audioElement = getAudioElement();
 
-      if (offset !== undefined) {
+      if (offset != undefined) {
         audioElement.currentTime = offset;
       }
 
diff --git a/src/store/dictionary.ts b/src/store/dictionary.ts
index c6a3108c14..d5e035520f 100644
--- a/src/store/dictionary.ts
+++ b/src/store/dictionary.ts
@@ -67,7 +67,7 @@ export const dictionaryStore = createPartialStore<DictionaryStoreTypes>({
       // 同じ単語IDで登録するために、１つのエンジンで登録したあと全エンジンに同期する。
       const engineId: EngineId | undefined = state.engineIds[0];
 
-      if (engineId === undefined)
+      if (engineId == undefined)
         throw new Error(`No such engine registered: index == 0`);
       await dispatch("INSTANTIATE_ENGINE_CONNECTOR", {
         engineId,
diff --git a/src/store/engine.ts b/src/store/engine.ts
index b20a5cc61c..b41c042f89 100644
--- a/src/store/engine.ts
+++ b/src/store/engine.ts
@@ -148,7 +148,7 @@ export const engineStore = createPartialStore<EngineStoreTypes>({
   IS_ENGINE_READY: {
     getter: (state) => (engineId) => {
       const engineState: EngineState | undefined = state.engineStates[engineId];
-      if (engineState === undefined)
+      if (engineState == undefined)
         throw new Error(`No such engineState set: engineId == ${engineId}`);
 
       return engineState === "READY";
@@ -159,12 +159,12 @@ export const engineStore = createPartialStore<EngineStoreTypes>({
     action: createUILockAction(
       async ({ state, commit, dispatch }, { engineId }) => {
         let engineState: EngineState | undefined = state.engineStates[engineId];
-        if (engineState === undefined)
+        if (engineState == undefined)
           throw new Error(`No such engineState set: engineId == ${engineId}`);
 
         for (let i = 0; i < 100; i++) {
           engineState = state.engineStates[engineId]; // FIXME: explicit undefined
-          if (engineState === undefined)
+          if (engineState == undefined)
             throw new Error(`No such engineState set: engineId == ${engineId}`);
 
           if (engineState === "FAILED_STARTING") {
@@ -268,7 +268,7 @@ export const engineStore = createPartialStore<EngineStoreTypes>({
   DETECTED_ENGINE_ERROR: {
     action({ state, commit }, { engineId }) {
       const engineState: EngineState | undefined = state.engineStates[engineId];
-      if (engineState === undefined)
+      if (engineState == undefined)
         throw new Error(`No such engineState set: engineId == ${engineId}`);
 
       switch (engineState) {
diff --git a/src/store/index.ts b/src/store/index.ts
index eca0e6dbba..271108925b 100644
--- a/src/store/index.ts
+++ b/src/store/index.ts
@@ -194,7 +194,7 @@ export const indexStore = createPartialStore<IndexStoreTypes>({
           const defaultStyleId = defaultStyleIds.find(
             (styleId) => styleId.speakerUuid == speakerUuid
           );
-          if (defaultStyleId === undefined) {
+          if (defaultStyleId == undefined) {
             return true;
           }
 
@@ -244,7 +244,7 @@ export const indexStore = createPartialStore<IndexStoreTypes>({
             audioItem.voice.styleId
           );
 
-          if (characterInfo === undefined)
+          if (characterInfo == undefined)
             throw new Error("assert characterInfo !== undefined");
 
           const speakerUuid = characterInfo.metas.speakerUuid;
diff --git a/src/store/preset.ts b/src/store/preset.ts
index 97fadd75fe..98817ee04f 100644
--- a/src/store/preset.ts
+++ b/src/store/preset.ts
@@ -134,9 +134,9 @@ export const presetStore = createPartialStore<PresetStoreTypes>({
 
       const presetConfig = await window.electron.getSetting("presets");
       if (
-        presetConfig === undefined ||
-        presetConfig.items === undefined ||
-        presetConfig.keys === undefined
+        presetConfig == undefined ||
+        presetConfig.items == undefined ||
+        presetConfig.keys == undefined
       )
         return;
       commit("SET_PRESET_ITEMS", {
diff --git a/src/store/project.ts b/src/store/project.ts
index 3c3053ee13..9152f7ebcd 100755
--- a/src/store/project.ts
+++ b/src/store/project.ts
@@ -175,7 +175,7 @@ export const projectStore = createPartialStore<ProjectStoreTypes>({
 
                 // set phoneme length
                 // 0.7 未満のプロジェクトファイルは styleId ではなく characterIndex なので、ここだけ characterIndex とした
-                if (audioItem.characterIndex === undefined)
+                if (audioItem.characterIndex == undefined)
                   throw new Error("audioItem.characterIndex === undefined");
                 await context
                   .dispatch("FETCH_MORA_DATA", {
@@ -227,7 +227,7 @@ export const projectStore = createPartialStore<ProjectStoreTypes>({
           ) {
             for (const audioItemsKey in projectData.audioItems) {
               const audioItem = projectData.audioItems[audioItemsKey];
-              if (audioItem.speaker !== null) {
+              if (audioItem.speaker != null) {
                 audioItem.styleId = audioItem.speaker;
                 delete audioItem.speaker;
               }
@@ -239,7 +239,7 @@ export const projectStore = createPartialStore<ProjectStoreTypes>({
           ) {
             for (const audioItemsKey in projectData.audioItems) {
               const audioItem = projectData.audioItems[audioItemsKey];
-              if (audioItem.engineId === undefined) {
+              if (audioItem.engineId == undefined) {
                 audioItem.engineId = engineId;
               }
             }
diff --git a/src/store/proxy.ts b/src/store/proxy.ts
index b10de3d7e3..26861d2931 100644
--- a/src/store/proxy.ts
+++ b/src/store/proxy.ts
@@ -15,7 +15,7 @@ const proxyStoreCreator = (_engineFactory: IEngineConnectorFactory) => {
       action({ state }, payload) {
         const engineId = payload.engineId;
         const engineInfo: EngineInfo | undefined = state.engineInfos[engineId];
-        if (engineInfo === undefined)
+        if (engineInfo == undefined)
           return Promise.reject(
             new Error(`No such engineInfo registered: engineId == ${engineId}`)
           );
diff --git a/src/store/utility.ts b/src/store/utility.ts
index 0c5d4ce5dc..6d1d0385de 100644
--- a/src/store/utility.ts
+++ b/src/store/utility.ts
@@ -145,7 +145,7 @@ function replaceTag(
 ): string {
   const result = template.replace(/\$(.+?)\$/g, (match, p1) => {
     const replaceTagId = replaceTagStringToTagId[p1];
-    if (replaceTagId === undefined) {
+    if (replaceTagId == undefined) {
       return match;
     }
     return replacer[replaceTagId] ?? "";
diff --git a/src/type/result.ts b/src/type/result.ts
index 49b9da9b77..1b2f2b2bf4 100644
--- a/src/type/result.ts
+++ b/src/type/result.ts
@@ -78,7 +78,7 @@ export const failure: Failure = <C extends string>(
       error: codeOrError,
     };
   } else if (codeOrError == undefined || typeof codeOrError === "string") {
-    if (error === undefined) {
+    if (error == undefined) {
       throw new Error("Error must be specified");
     }
     return { ok: false as const, code: codeOrError, error };
diff --git a/tests/unit/lib/map.spec.ts b/tests/unit/lib/map.spec.ts
index dd5c93a1ad..57ef246c48 100644
--- a/tests/unit/lib/map.spec.ts
+++ b/tests/unit/lib/map.spec.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-import { mapUndefinedPipe, undefinedToDefault } from "@/helpers/map";
+import { mapNullablePipe, nullableToDefault } from "@/helpers/map";
 
 type DummyType = {
   outerValue?: {
@@ -19,7 +19,7 @@ describe("mapUndefinedPipe", () => {
     };
     const map = new Map<string, DummyType>([[key, value]]);
     expect(
-      mapUndefinedPipe(
+      mapNullablePipe(
         map.get(key),
         (v) => v.outerValue,
         (v) => v.innerValue
@@ -36,7 +36,7 @@ describe("mapUndefinedPipe", () => {
     };
     const map = new Map<string, DummyType>([[key, value]]);
     expect(
-      mapUndefinedPipe(
+      mapNullablePipe(
         map.get(key),
         (v) => v.outerValue,
         (v) => v.innerValue
@@ -48,16 +48,11 @@ describe("mapUndefinedPipe", () => {
 describe("undefinedToDefault", () => {
   it("値がある時はそのまま返す", () => {
     const actualValue = "value";
-    expect(undefinedToDefault("test", actualValue)).toEqual(actualValue);
+    expect(nullableToDefault("test", actualValue)).toEqual(actualValue);
   });
 
   it("値がない時はdefaultValueを返す", () => {
     const defaultValue = "test";
-    expect(undefinedToDefault(defaultValue, undefined)).toEqual(defaultValue);
-  });
-
-  it("undefinedのみを値がない状態とみなす", () => {
-    const defaultValue = "test";
-    expect(undefinedToDefault(defaultValue, null)).toBeNull();
+    expect(nullableToDefault(defaultValue, undefined)).toEqual(defaultValue);
   });
 });