diff --git a/src/main/java/net/javahippie/fitpub/repository/ActivityRepository.java b/src/main/java/net/javahippie/fitpub/repository/ActivityRepository.java
index 910c66e..f0cc31d 100644
--- a/src/main/java/net/javahippie/fitpub/repository/ActivityRepository.java
+++ b/src/main/java/net/javahippie/fitpub/repository/ActivityRepository.java
@@ -48,9 +48,15 @@ public interface ActivityRepository extends JpaRepository<Activity, UUID> {
      * @return list of activities
      */
     List<Activity> findByUserIdAndStartedAtBetweenOrderByStartedAtDesc(
-        UUID userId,
-        LocalDateTime startDate,
-        LocalDateTime endDate
+            UUID userId,
+            LocalDateTime startDate,
+            LocalDateTime endDate
+    );
+
+    boolean existsByUserIdAndStartedAtBetween(
+            UUID userId,
+            LocalDateTime startDate,
+            LocalDateTime endDate
     );
 
     /**
diff --git a/src/main/java/net/javahippie/fitpub/service/ActivitySummaryService.java b/src/main/java/net/javahippie/fitpub/service/ActivitySummaryService.java
index 6579e45..fd054ac 100644
--- a/src/main/java/net/javahippie/fitpub/service/ActivitySummaryService.java
+++ b/src/main/java/net/javahippie/fitpub/service/ActivitySummaryService.java
@@ -229,26 +229,67 @@ public class ActivitySummaryService {
     /**
      * Get current week summary.
      */
-    @Transactional(readOnly = true)
+    @Transactional
     public ActivitySummary getCurrentWeekSummary(UUID userId) {
         LocalDate weekStart = LocalDate.now().with(TemporalAdjusters.previousOrSame(java.time.DayOfWeek.MONDAY));
-        return activitySummaryRepository.findByUserIdAndPeriodTypeAndPeriodStart(
+        LocalDate weekEndExclusive = weekStart.plusDays(7);
+        return getOrBuildCurrentSummary(
                 userId,
                 ActivitySummary.PeriodType.WEEK,
-                weekStart
-        ).orElse(null);
+                weekStart,
+                weekEndExclusive,
+                () -> updateWeeklySummary(userId, weekStart)
+        );
     }
 
     /**
      * Get current month summary.
      */
-    @Transactional(readOnly = true)
+    @Transactional
     public ActivitySummary getCurrentMonthSummary(UUID userId) {
         LocalDate monthStart = LocalDate.now().with(TemporalAdjusters.firstDayOfMonth());
-        return activitySummaryRepository.findByUserIdAndPeriodTypeAndPeriodStart(
+        LocalDate monthEndExclusive = monthStart.plusMonths(1);
+        return getOrBuildCurrentSummary(
                 userId,
                 ActivitySummary.PeriodType.MONTH,
-                monthStart
+                monthStart,
+                monthEndExclusive,
+                () -> updateMonthlySummary(userId, monthStart)
+        );
+    }
+
+    private ActivitySummary getOrBuildCurrentSummary(
+            UUID userId,
+            ActivitySummary.PeriodType periodType,
+            LocalDate periodStart,
+            LocalDate periodEndExclusive,
+            Runnable rebuildAction
+    ) {
+        ActivitySummary existingSummary = activitySummaryRepository.findByUserIdAndPeriodTypeAndPeriodStart(
+                userId,
+                periodType,
+                periodStart
+        ).orElse(null);
+
+        if (existingSummary != null) {
+            return existingSummary;
+        }
+
+        boolean hasActivitiesInPeriod = activityRepository.existsByUserIdAndStartedAtBetween(
+                userId,
+                periodStart.atStartOfDay(),
+                periodEndExclusive.atStartOfDay()
+        );
+
+        if (!hasActivitiesInPeriod) {
+            return null;
+        }
+
+        rebuildAction.run();
+        return activitySummaryRepository.findByUserIdAndPeriodTypeAndPeriodStart(
+                userId,
+                periodType,
+                periodStart
         ).orElse(null);
     }
 }
diff --git a/src/test/java/net/javahippie/fitpub/service/ActivitySummaryServiceTest.java b/src/test/java/net/javahippie/fitpub/service/ActivitySummaryServiceTest.java
index fdccc74..bab62b8 100644
--- a/src/test/java/net/javahippie/fitpub/service/ActivitySummaryServiceTest.java
+++ b/src/test/java/net/javahippie/fitpub/service/ActivitySummaryServiceTest.java
@@ -21,6 +21,8 @@ import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
 
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -92,4 +94,72 @@ class ActivitySummaryServiceTest {
                 summary.getActivityCount() == 1
         ));
     }
+
+    @Test
+    @DisplayName("Should rebuild current month summary on demand when activities exist but summary is missing")
+    void getCurrentMonthSummary_RebuildsMissingSummary() {
+        LocalDate monthStart = LocalDate.now().withDayOfMonth(1);
+        LocalDateTime startDateTime = monthStart.atStartOfDay();
+        LocalDateTime endDateTime = monthStart.plusMonths(1).atStartOfDay();
+
+        Activity activity = Activity.builder()
+                .id(UUID.randomUUID())
+                .userId(userId)
+                .activityType(Activity.ActivityType.RUN)
+                .startedAt(startDateTime.plusDays(2).plusHours(7))
+                .endedAt(startDateTime.plusDays(2).plusHours(8))
+                .totalDistance(BigDecimal.valueOf(10000))
+                .totalDurationSeconds(3600L)
+                .elevationGain(BigDecimal.valueOf(150))
+                .build();
+
+        ActivitySummary rebuiltSummary = ActivitySummary.builder()
+                .userId(userId)
+                .periodType(ActivitySummary.PeriodType.MONTH)
+                .periodStart(monthStart)
+                .periodEnd(monthStart.plusMonths(1).minusDays(1))
+                .activityCount(1)
+                .build();
+
+        when(activitySummaryRepository.findByUserIdAndPeriodTypeAndPeriodStart(
+                userId,
+                ActivitySummary.PeriodType.MONTH,
+                monthStart
+        )).thenReturn(Optional.empty(), Optional.of(rebuiltSummary));
+        when(activityRepository.existsByUserIdAndStartedAtBetween(userId, startDateTime, endDateTime))
+                .thenReturn(true);
+        when(activityRepository.findByUserIdAndStartedAtBetweenOrderByStartedAtDesc(userId, startDateTime, endDateTime))
+                .thenReturn(List.of(activity));
+        when(personalRecordRepository.countByUserIdAndDateRange(userId, startDateTime, endDateTime)).thenReturn(0L);
+        when(achievementRepository.countByUserIdAndActivityStartedDateRange(userId, startDateTime, endDateTime)).thenReturn(0L);
+        when(activitySummaryRepository.save(org.mockito.ArgumentMatchers.any(ActivitySummary.class)))
+                .thenAnswer(invocation -> invocation.getArgument(0));
+
+        ActivitySummary result = activitySummaryService.getCurrentMonthSummary(userId);
+
+        assertNotNull(result);
+        assertEquals(1, result.getActivityCount());
+        verify(activityRepository).existsByUserIdAndStartedAtBetween(userId, startDateTime, endDateTime);
+    }
+
+    @Test
+    @DisplayName("Should return null for current month summary when no activities exist in the period")
+    void getCurrentMonthSummary_ReturnsNullWhenNoActivitiesExist() {
+        LocalDate monthStart = LocalDate.now().withDayOfMonth(1);
+        LocalDateTime startDateTime = monthStart.atStartOfDay();
+        LocalDateTime endDateTime = monthStart.plusMonths(1).atStartOfDay();
+
+        when(activitySummaryRepository.findByUserIdAndPeriodTypeAndPeriodStart(
+                userId,
+                ActivitySummary.PeriodType.MONTH,
+                monthStart
+        )).thenReturn(Optional.empty());
+        when(activityRepository.existsByUserIdAndStartedAtBetween(userId, startDateTime, endDateTime))
+                .thenReturn(false);
+
+        ActivitySummary result = activitySummaryService.getCurrentMonthSummary(userId);
+
+        assertNull(result);
+        verify(activityRepository).existsByUserIdAndStartedAtBetween(userId, startDateTime, endDateTime);
+    }
 }