diff --git a/src/main/java/org/ktc2/cokaen/wouldyouin/event/application/EventService.java b/src/main/java/org/ktc2/cokaen/wouldyouin/event/application/EventService.java
index 879f7759..65f34c23 100644
--- a/src/main/java/org/ktc2/cokaen/wouldyouin/event/application/EventService.java
+++ b/src/main/java/org/ktc2/cokaen/wouldyouin/event/application/EventService.java
@@ -132,11 +132,13 @@ public void delete(MemberIdentifier identifier, Long eventId) {
     }
 
     @Transactional
-    public void decreaseLeftSeat(Long eventId, Integer count) {
+    public void changeLeftSeat(Long eventId, Integer count) {
         Event event = getByIdOrThrow(eventId);
         if (event.getLeftSeat() < count) {
             throw new NoLeftSeatException("남은 좌석이 부족합니다.");
         }
+        if (event.getLeftSeat() > event.getTotalSeat())
+            throw new NoLeftSeatException("좌석 관련 알 수 없는 오류");
         event.decreaseLeftSeat(count);
     }
 
diff --git a/src/main/java/org/ktc2/cokaen/wouldyouin/event/persist/Event.java b/src/main/java/org/ktc2/cokaen/wouldyouin/event/persist/Event.java
index 050f852d..1d06a319 100644
--- a/src/main/java/org/ktc2/cokaen/wouldyouin/event/persist/Event.java
+++ b/src/main/java/org/ktc2/cokaen/wouldyouin/event/persist/Event.java
@@ -147,6 +147,6 @@ public void updateFrom(EventEditRequest eventEditRequest, List<EventImage> image
     }
 
     public void decreaseLeftSeat(Integer count) {
-        this.leftSeat -= count;
+        this.leftSeat += count;
     }
 }
\ No newline at end of file
diff --git a/src/main/java/org/ktc2/cokaen/wouldyouin/reservation/application/ReservationService.java b/src/main/java/org/ktc2/cokaen/wouldyouin/reservation/application/ReservationService.java
index b3127927..557a22fd 100644
--- a/src/main/java/org/ktc2/cokaen/wouldyouin/reservation/application/ReservationService.java
+++ b/src/main/java/org/ktc2/cokaen/wouldyouin/reservation/application/ReservationService.java
@@ -6,19 +6,14 @@
 import org.ktc2.cokaen.wouldyouin._common.exception.UnauthorizedException;
 import org.ktc2.cokaen.wouldyouin.auth.MemberIdentifier;
 import org.ktc2.cokaen.wouldyouin.event.application.EventService;
+import org.ktc2.cokaen.wouldyouin.event.persist.Event;
 import org.ktc2.cokaen.wouldyouin.member.application.MemberService;
-import org.ktc2.cokaen.wouldyouin.payment.application.PaymentService;
-import org.ktc2.cokaen.wouldyouin.payment.dto.KakaoPayRequest;
-import org.ktc2.cokaen.wouldyouin.payment.dto.KakaoPayResponse;
-import org.ktc2.cokaen.wouldyouin.reservation.api.dto.KakaoPayReservationResponse;
 import org.ktc2.cokaen.wouldyouin.reservation.api.dto.ReservationRequest;
 import org.ktc2.cokaen.wouldyouin.reservation.api.dto.ReservationResponse;
 import org.ktc2.cokaen.wouldyouin.reservation.api.dto.ReservationSliceResponse;
 import org.ktc2.cokaen.wouldyouin.reservation.exception.ReservationNotFoundForReviewException;
 import org.ktc2.cokaen.wouldyouin.reservation.persist.Reservation;
 import org.ktc2.cokaen.wouldyouin.reservation.persist.ReservationRepository;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 import org.springframework.data.domain.Pageable;
 import org.springframework.data.domain.Slice;
 import org.springframework.stereotype.Service;
@@ -63,8 +58,8 @@ public ReservationResponse create(
             memberService.getByIdOrThrow(memberId),
             eventService.getByIdOrThrow(reservationRequest.getEventId()))
         );
-        eventService.decreaseLeftSeat(reservation.getEvent().getId(),
-            reservationRequest.getQuantity());
+        eventService.changeLeftSeat(reservation.getEvent().getId(),
+            reservationRequest.getQuantity() * -1);
         return ReservationResponse.from(reservation);
     }
 
@@ -74,16 +69,16 @@ public ReservationResponse createTest(
         Reservation reservation = reservationRepository.save(reservationRequest.toEntity(
             memberService.getByIdOrThrow(identifier.id()),
             eventService.getByIdOrThrow(reservationRequest.getEventId())));
-        eventService.decreaseLeftSeat(reservation.getEvent().getId(),
-            reservationRequest.getQuantity());
+        eventService.changeLeftSeat(reservation.getEvent().getId(),
+            reservationRequest.getQuantity() * -1);
         return ReservationResponse.from(reservation);
     }
 
     @Transactional
     public void delete(MemberIdentifier identifier, Long reservationId) {
         validateMemberId(identifier.id(), getByIdOrThrow(reservationId));
-        eventService.decreaseLeftSeat(-getByIdOrThrow(reservationId).getEvent().getId(),
-            getByIdOrThrow(reservationId).getQuantity());
+        Event event = getByIdOrThrow(reservationId).getEvent();
+        eventService.changeLeftSeat(event.getId(), getByIdOrThrow(reservationId).getQuantity());
         reservationRepository.deleteById(reservationId);
     }