diff --git a/src/sys/ags.h b/src/sys/ags.h
index 983a1bb..8fe066e 100644
--- a/src/sys/ags.h
+++ b/src/sys/ags.h
@@ -124,7 +124,7 @@ public:
 	void box_fill(int dest, int sx, int sy, int ex, int ey, uint8 color);
 	void box_line(int dest, int sx, int sy, int ex, int ey, uint8 color);
 
-	void draw_text(char string[]);
+	void draw_text(const char* string);
 
 	void clear_text_window(int index, bool erase);
 	bool return_text_line(int index);
diff --git a/src/sys/ags_text.cpp b/src/sys/ags_text.cpp
index 6a30525..573a49a 100644
--- a/src/sys/ags_text.cpp
+++ b/src/sys/ags_text.cpp
@@ -11,7 +11,7 @@
 
 static bool antialias = false;
 
-void AGS::draw_text(char string[])
+void AGS::draw_text(const char* string)
 {
 	int p = 0;
 	int screen, dest_x, dest_y, font_size;
diff --git a/src/sys/nact_sys1.cpp b/src/sys/nact_sys1.cpp
index 658ae71..aff3313 100644
--- a/src/sys/nact_sys1.cpp
+++ b/src/sys/nact_sys1.cpp
@@ -361,7 +361,7 @@ void NACT::cmd_open_menu()
 			if(terminate) {
 				return;
 			}
-			if(val = get_key()) {
+			if((val = get_key())) {
 				SDL_Delay(100);
 				break;
 			}
@@ -565,7 +565,7 @@ top2:
 			if(terminate) {
 				return;
 			}
-			if(val = get_key()) {
+			if((val = get_key())) {
 				SDL_Delay(100);
 				break;
 			}
@@ -753,7 +753,7 @@ top2:
 			if(terminate) {
 				return;
 			}
-			if(val = get_key()) {
+			if((val = get_key())) {
 				SDL_Delay(100);
 				break;
 			}
@@ -1435,7 +1435,7 @@ void NACT::cmd_z()
 	switch (crc32) {
 	case CRC32_INTRUDER:
 		if(cmd == 1 && 1 <= param && param <= 4) {
-			char* buf[4] = {"＜", "∧", "＞", "∨"};
+			const char* buf[4] = {"＜", "∧", "＞", "∨"};
 
 			// 矢印を消去する
 			if(map_page) {
diff --git a/src/sys/nact_sys2.cpp b/src/sys/nact_sys2.cpp
index 89abe6d..e9e39df 100644
--- a/src/sys/nact_sys2.cpp
+++ b/src/sys/nact_sys2.cpp
@@ -391,7 +391,7 @@ void NACT::cmd_open_menu()
 			if(terminate) {
 				return;
 			}
-			if(val = get_key()) {
+			if((val = get_key())) {
 				SDL_Delay(100);
 				break;
 			}
@@ -560,7 +560,7 @@ void NACT::cmd_open_verb()
 			if(terminate) {
 				return;
 			}
-			if(val = get_key()) {
+			if((val = get_key())) {
 				SDL_Delay(100);
 				break;
 			}
@@ -704,7 +704,7 @@ void NACT::cmd_open_obj(int verb)
 			if(terminate) {
 				return;
 			}
-			if(val = get_key()) {
+			if((val = get_key())) {
 				SDL_Delay(100);
 				break;
 			}
diff --git a/src/sys/nact_sys3.cpp b/src/sys/nact_sys3.cpp
index a489fc0..708da8d 100644
--- a/src/sys/nact_sys3.cpp
+++ b/src/sys/nact_sys3.cpp
@@ -195,7 +195,7 @@ int NACT::menu_select(int num_items)
 			int dy = my - current_my;
 			if (dx*dx + dy*dy > 10)
 				break;
-			if(val = get_key()) {
+			if((val = get_key())) {
 				SDL_Delay(100);
 				break;
 			}
@@ -703,7 +703,7 @@ void NACT::cmd_k()
 		if(terminate) {
 			return;
 		}
-		if(val = get_key()) {
+		if((val = get_key())) {
 			break;
 		}
 		if(1 <= cmd && cmd <= 3) {
@@ -1448,7 +1448,7 @@ void NACT::cmd_y()
 				get_cursor(&x, &y);
 				M_X = (x < 0) ? x : (x > 639) ? 639 : x;
 				M_Y = (y < 0) ? 0 : (y > 479) ? 479 : y;
-				if(RND = get_key()) {
+				if((RND = get_key())) {
 					// キー入力があった場合は、離すまで待機
 					for(;;) {
 						if(terminate) {