diff --git a/src/ags.c b/src/ags.c
index 7e022b5..0a26e4f 100644
--- a/src/ags.c
+++ b/src/ags.c
@@ -822,11 +822,6 @@ boolean ags_getAntialiasedStringMode() {
 	return font_get_antialias();
 }
 
-void ags_fullscreen(boolean on) {
-	nact->sys_fullscreen_on = on;
-	sdl_FullScreen(on);
-}
-
 void ags_copyArea_shadow_withrate(int sx, int sy, int w, int h, int dx, int dy, int lv) {
 	if (nact->sys_world_depth == 8) return;
 	
diff --git a/src/ags.h b/src/ags.h
index 3c188b4..b346101 100644
--- a/src/ags.h
+++ b/src/ags.h
@@ -149,7 +149,6 @@ extern void ags_getDIBInfo(DispInfo *info);
 extern void ags_getWindowInfo(DispInfo *info);
 extern void ags_getViewAreaInfo(DispInfo *info);
 extern boolean ags_regionContains(MyRectangle *r, int x, int y);
-extern void    ags_fullscreen(boolean on);
 extern boolean ags_check_param(int *x, int *y, int *w, int *h);
 extern boolean ags_check_param_xy(int *x, int *y);
 extern void    ags_intersection(MyRectangle *r1, MyRectangle *r2, MyRectangle *rst);
diff --git a/src/cmdz.c b/src/cmdz.c
index b72d1ac..8099669 100644
--- a/src/cmdz.c
+++ b/src/cmdz.c
@@ -429,7 +429,7 @@ void commandZZ10() {
 	/* スクリーンモードを取得する */
 	int *var = getCaliVariable();
 
-	*var = nact->sys_fullscreen_on ? 1 : 0;
+	*var = sdl_isFullscreen() ? 1 : 0;
 	
 	DEBUG_COMMAND("ZZ10 %d:\n",*var);
 }
diff --git a/src/nact.h b/src/nact.h
index e519106..a564796 100644
--- a/src/nact.h
+++ b/src/nact.h
@@ -78,7 +78,6 @@ typedef struct {
 	void *datatbl_addr; /* データテーブルのアドレス */
 	int fnc_return_value; /* 関数の戻り値として返す値 (~0,cali:で渡す値) */
 	
-#if 1
 	/* ags info */
 	Palette256 *sys_pal;
 	boolean     sys_pal_changed;
@@ -86,10 +85,6 @@ typedef struct {
 	MyDimension sys_world_size;
 	int         sys_world_depth;
 	int         sys_mouse_movesw; /* 0:IZを無視, 1: 直接指定場所へ, 2: スムーズに指定場所に */
-	boolean     sys_fullscreen_capable;
-	boolean     sys_fullscreen_on;
-	
-#endif
 
 	/* for fader/ecopy */
 	int     effect_rate;
diff --git a/src/sdl_core.h b/src/sdl_core.h
index 52d78f6..beddb9d 100644
--- a/src/sdl_core.h
+++ b/src/sdl_core.h
@@ -41,12 +41,12 @@ extern void sdl_setWorldSize(int width, int height, int depth);
 extern void sdl_setWindowSize(int x, int y, int w, int h);
 extern void sdl_setWindowTitle(char *name);
 extern void sdl_getWindowInfo(DispInfo *info);
-extern void sdl_FullScreen(boolean on);
+extern void sdl_setFullscreen(boolean on);
+extern boolean sdl_isFullscreen(void);
 extern agsurface_t *sdl_getDIB(void);
 
 /* 画面更新 */
 extern void sdl_updateArea(MyRectangle *src, MyPoint *dst);
-extern void sdl_fullScreen(boolean on);
 extern void sdl_updateScreen(void);
 
 /* パレット関係 */
diff --git a/src/sdl_event.c b/src/sdl_event.c
index d0cb201..740ed8f 100644
--- a/src/sdl_event.c
+++ b/src/sdl_event.c
@@ -139,13 +139,14 @@ static void sdl_getEvent(void) {
 			break;
 		case SDL_KEYUP:
 			keyEventProsess(&e.key, FALSE);
-			if (e.key.keysym.sym == SDLK_F1) msg_skip = TRUE;
-#ifndef __EMSCRIPTEN__
-			if (e.key.keysym.sym == SDLK_F4) {
-				sdl_fs_on = !sdl_fs_on;
-				SDL_SetWindowFullscreen(sdl_window, sdl_fs_on ? SDL_WINDOW_FULLSCREEN_DESKTOP : 0);
+			switch (e.key.keysym.sym) {
+			case SDLK_F1:
+				msg_skip = TRUE;
+				break;
+			case SDLK_F4:
+				sdl_setFullscreen(!sdl_fs_on);
+				break;
 			}
-#endif
 			break;
 		case SDL_MOUSEMOTION:
 			mousex = e.motion.x;
diff --git a/src/sdl_video.c b/src/sdl_video.c
index 29bd763..66ddf58 100644
--- a/src/sdl_video.c
+++ b/src/sdl_video.c
@@ -201,14 +201,17 @@ void sdl_setAutoRepeat(boolean bool) {
 	}
 }
 
-void sdl_FullScreen(boolean on) {
-	if (on && !sdl_fs_on) {
-		sdl_fs_on = TRUE;
-		SDL_SetWindowFullscreen(sdl_window, SDL_WINDOW_FULLSCREEN_DESKTOP);
-	} else if (!on && sdl_fs_on) {
-		sdl_fs_on = FALSE;
-		SDL_SetWindowFullscreen(sdl_window, 0);
-	}
+void sdl_setFullscreen(boolean on) {
+#ifndef __EMSCRIPTEN__
+	if (on == sdl_fs_on)
+		return;
+	SDL_SetWindowFullscreen(sdl_window, on ? SDL_WINDOW_FULLSCREEN_DESKTOP: 0);
+	sdl_fs_on = on;
+#endif
+}
+
+boolean sdl_isFullscreen(void) {
+	return sdl_fs_on;
 }
 
 void sdl_setWindowSize(int x, int y, int w, int h) {
diff --git a/src/win/menubar.c b/src/win/menubar.c
index dbd603d..0327f30 100644
--- a/src/win/menubar.c
+++ b/src/win/menubar.c
@@ -22,9 +22,9 @@
 #include <SDL_syswm.h>
 #include "system.h"
 #include "menu.h"
+#include "sdl_core.h"
 #include "sdl_private.h"
 #include "resources.h"
-#include "ags.h"
 
 static HWND get_hwnd(SDL_Window *window) {
 	SDL_SysWMinfo info;
@@ -50,10 +50,10 @@ void win_menu_onsyswmevent(SDL_SysWMmsg* msg) {
 			menu_quitmenu_open();
 			break;
 		case ID_SCREEN_WINDOW:
-			ags_fullscreen(FALSE);
+			sdl_setFullscreen(FALSE);
 			break;
 		case ID_SCREEN_FULL:
-			ags_fullscreen(TRUE);
+			sdl_setFullscreen(TRUE);
 			break;
 		}
 		break;
diff --git a/src/xsystem35.c b/src/xsystem35.c
index 2be0f6d..d77b389 100644
--- a/src/xsystem35.c
+++ b/src/xsystem35.c
@@ -220,7 +220,7 @@ static void sys35_init() {
 	for (i = 0; i < FONTTYPEMAX; i++)
 		font_set_name_and_index(i, fontname_tt[i], fontface[i]);
 	
-	ags_fullscreen(fs_on);
+	sdl_setFullscreen(fs_on);
 	nact->noantialias = font_noantialias;
 	ags_setAntialiasedStringMode(!font_noantialias);