diff --git a/modules/Gpx/surface.c b/modules/Gpx/surface.c
index a8d4e79..1dbde3d 100644
--- a/modules/Gpx/surface.c
+++ b/modules/Gpx/surface.c
@@ -11,11 +11,9 @@
 
 static surface_t *create(int width, int height, bool has_pixel, bool has_alpha) {
 	surface_t *s = calloc(1, sizeof(surface_t));
-	
 	s->width = width;
 	s->height = height;
-	s->depth = 24;
-	
+
 	if (has_pixel) {
 		s->sdl_surface = SDL_CreateRGBSurfaceWithFormat(0, width, height, 32, SDL_PIXELFORMAT_RGB888);
 	}
diff --git a/modules/lib/graph_blend_amap.c b/modules/lib/graph_blend_amap.c
index 9f20be8..261fbc8 100644
--- a/modules/lib/graph_blend_amap.c
+++ b/modules/lib/graph_blend_amap.c
@@ -13,7 +13,7 @@ static void gre_BlendUseAMap(surface_t *dst, int dx, int dy, surface_t *src, int
 	uint8_t *dp = GETOFFSET_PIXEL(dst, dx, dy);
 	uint8_t *ap = GETOFFSET_ALPHA(src, sx, sy);
 
-	switch(dst->depth) {
+	switch (dst->sdl_surface->format->BitsPerPixel) {
 	case 16:
 		for (int y = 0; y < height; y++) {
 			uint16_t *yls = (uint16_t *)(sp + y * src->sdl_surface->pitch);
diff --git a/src/ags.h b/src/ags.h
index 81a97d8..3d58eef 100644
--- a/src/ags.h
+++ b/src/ags.h
@@ -61,12 +61,11 @@ typedef enum {
 } ALPHA_DIB_COPY_TYPE;
 
 struct agsurface {
-	int width;   /* width of surface  */
-	int height;  /* height of surface */
-	int depth;   /* depth of surface, 8/16/24/32 is available */
+	int width;
+	int height;
 	
-	SDL_Surface *sdl_surface; /* pixel data (can be NULL) */
-	uint8_t *alpha; /* alpha pixel data (can be NULL) */
+	SDL_Surface *sdl_surface; // pixel data (can be NULL)
+	uint8_t *alpha; // alpha data (can be NULL)
 };
 typedef struct agsurface surface_t;
 
diff --git a/src/sdl_image.c b/src/sdl_image.c
index 2d5f122..cef1882 100644
--- a/src/sdl_image.c
+++ b/src/sdl_image.c
@@ -110,7 +110,7 @@ void sdl_drawImage16(cgdata *cg, surface_t *sf, int dx, int dy, int brightness,
 				*dst++ = rgb565_to_rgb888(*p_src++) | *a_src++ << 24;
 			}
 		}
-	} else if (sf->depth > 16) {
+	} else if (sf->sdl_surface->format->BitsPerPixel > 16) {
 		// Convert to RGB888 by ourselves. SDL blit functions use a slightly
 		// different mapping, so the color expanded by SDL may not match the
 		// color key specified in the CX command.
diff --git a/src/sdl_video.c b/src/sdl_video.c
index fc7c502..bd12b38 100644
--- a/src/sdl_video.c
+++ b/src/sdl_video.c
@@ -191,13 +191,12 @@ static void makeDIB(int width, int height, int depth) {
 	}
 	
 	sdl_dibinfo = calloc(1, sizeof(surface_t));
-	sdl_dibinfo->depth           = main_surface->format->BitsPerPixel;
 	sdl_dibinfo->width  = width;
 	sdl_dibinfo->height = height;
 	sdl_dibinfo->alpha  = NULL;
 	sdl_dibinfo->sdl_surface = main_surface;
 	
-	image_setdepth(sdl_dibinfo->depth);
+	image_setdepth(main_surface->format->BitsPerPixel);
 }
 
 /* offscreen の設定 */