diff --git a/src/main/java/com/gdschongik/gdsc/domain/study/application/MentorStudyAchievementService.java b/src/main/java/com/gdschongik/gdsc/domain/study/application/MentorStudyAchievementService.java
index efb0de43b..c639129a5 100644
--- a/src/main/java/com/gdschongik/gdsc/domain/study/application/MentorStudyAchievementService.java
+++ b/src/main/java/com/gdschongik/gdsc/domain/study/application/MentorStudyAchievementService.java
@@ -58,7 +58,7 @@ public void designateOutstandingStudent(Long studyId, OutstandingStudentRequest
     public void withdrawOutstandingStudent(Long studyId, OutstandingStudentRequest request) {
         Member currentMember = memberUtil.getCurrentMember();
         Study study = studyRepository.findById(studyId).orElseThrow(() -> new CustomException(STUDY_NOT_FOUND));
-        Long countByStudyIdAndStudentIds =
+        long countByStudyIdAndStudentIds =
                 studyHistoryRepository.countByStudyIdAndStudentIds(studyId, request.studentIds());
 
         studyValidator.validateStudyMentor(currentMember, study);
diff --git a/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepository.java b/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepository.java
index 4ec8b10bc..f64f7be6a 100644
--- a/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepository.java
+++ b/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepository.java
@@ -4,5 +4,5 @@
 
 public interface StudyHistoryCustomRepository {
 
-    Long countByStudyIdAndStudentIds(Long studyId, List<Long> studentIds);
+    long countByStudyIdAndStudentIds(Long studyId, List<Long> studentIds);
 }
diff --git a/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepositoryImpl.java b/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepositoryImpl.java
index e827b330f..32232e65c 100644
--- a/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepositoryImpl.java
+++ b/src/main/java/com/gdschongik/gdsc/domain/study/dao/StudyHistoryCustomRepositoryImpl.java
@@ -13,8 +13,8 @@ public class StudyHistoryCustomRepositoryImpl implements StudyHistoryCustomRepos
     private final JPAQueryFactory queryFactory;
 
     @Override
-    public Long countByStudyIdAndStudentIds(Long studyId, List<Long> studentIds) {
-        return queryFactory
+    public long countByStudyIdAndStudentIds(Long studyId, List<Long> studentIds) {
+        return (long) queryFactory
                 .select(studyHistory.count())
                 .from(studyHistory)
                 .where(eqStudyId(studyId), studyHistory.student.id.in(studentIds))
diff --git a/src/main/java/com/gdschongik/gdsc/domain/study/domain/StudyHistoryValidator.java b/src/main/java/com/gdschongik/gdsc/domain/study/domain/StudyHistoryValidator.java
index 391925c89..6a21b0f44 100644
--- a/src/main/java/com/gdschongik/gdsc/domain/study/domain/StudyHistoryValidator.java
+++ b/src/main/java/com/gdschongik/gdsc/domain/study/domain/StudyHistoryValidator.java
@@ -52,11 +52,7 @@ public void validateUpdateRepository(
         }
     }
 
-    public void validateAppliedToStudy(Long countStudyHistory, int studentCount) {
-        if (countStudyHistory == null) {
-            throw new CustomException(STUDY_HISTORY_NOT_FOUND);
-        }
-
+    public void validateAppliedToStudy(long countStudyHistory, int studentCount) {
         if (countStudyHistory != studentCount) {
             throw new CustomException(STUDY_HISTORY_NOT_APPLIED_STUDENT_EXISTS);
         }