diff --git a/bg.jpg b/bg.jpg
new file mode 100644
index 0000000..b4b3b73
Binary files /dev/null and b/bg.jpg differ
diff --git a/main.py b/main.py
index 04810ba..3d9d6dd 100644
--- a/main.py
+++ b/main.py
@@ -64,6 +64,11 @@ class Main:
         if goatoh_mode:
             balloon = pygame.transform.flip (balloon, False, True)
 
+        # 背景（昼）
+        bg_day: pygame.Surface = pygame.transform.scale (
+                pygame.image.load ('bg.jpg'),
+                (CWindow.WIDTH, CWindow.HEIGHT))
+
         # 背景（夕方）
         bg_evening: pygame.Surface = pygame.transform.scale (
                 pygame.image.load ('bg-evening.jpg'),
@@ -124,7 +129,7 @@ class Main:
             # 日の入終了
             sunset_end: datetime = sunset_start + timedelta (hours = 1)
 
-            cls.draw_bg (screen, bg_evening, bg_night,
+            cls.draw_bg (screen, bg_day, bg_evening, bg_night,
                          sunrise_start, sunrise_end, sunset_start, sunset_end)
 
             # 左上に時刻表示
@@ -248,7 +253,7 @@ class Main:
                         if not double_mode or random.random () < .5:
                             break
 
-                        cls.draw_bg (screen, bg_evening, bg_night,
+                        cls.draw_bg (screen, bg_day, bg_evening, bg_night,
                                      sunrise_start, sunrise_end,
                                      sunset_start, sunset_end)
 
@@ -277,6 +282,7 @@ class Main:
     def draw_bg (
             cls,
             screen:         pygame.Surface,
+            bg_day:         pygame.Surface,
             bg_evening:     pygame.Surface,
             bg_night:       pygame.Surface,
             sunrise_start:  datetime,
@@ -284,23 +290,28 @@ class Main:
             sunset_start:   datetime,
             sunset_end:     datetime) \
     -> None:
+        sunrise_centre: datetime = (
+                sunrise_start + (sunrise_end - sunrise_start) / 2)
+        sunset_centre: datetime = (
+                sunset_start + (sunset_end - sunset_start) / 2)
+
         dt: datetime = datetime.now ()
 
-        if (dt < sunrise_end) or (sunset_start <= dt):
+        if sunrise_centre <= dt < sunset_centre:
+            screen.blit (bg_day, (0, 0))
+        else:
             screen.blit (bg_night, (0, 0))
 
-            if sunrise_start <= dt < sunrise_end:
-                bg_evening.set_alpha (((dt - sunrise_start) * 255)
-                                      / (sunrise_end - sunrise_start))
-            elif sunset_start <= dt < sunset_end:
-                bg_evening.set_alpha (((sunset_end - dt) * 255)
-                                      / (sunset_end - sunset_start))
-            else:
-                bg_evening.set_alpha (0)
-
-            screen.blit (bg_evening, (0, 0))
+        if sunrise_start <= dt < sunrise_end:
+            bg_evening.set_alpha (255 - ((abs (dt - sunrise_centre) * 510)
+                                         / (sunrise_end - sunrise_centre)))
+        elif sunset_start <= dt < sunset_end:
+            bg_evening.set_alpha (255 - ((abs (dt - sunset_centre) * 510)
+                                         / (sunset_end - sunset_centre)))
         else:
-            screen.fill ((0, 255, 0))
+            return
+
+        screen.blit (bg_evening, (0, 0))
 
 
 if __name__ == '__main__':