diff --git a/src/common.h b/src/common.h
index 175936a..0e32d87 100644
--- a/src/common.h
+++ b/src/common.h
@@ -65,6 +65,14 @@ inline void strcpy_s(char* dst, size_t n, const char* src)
 
 #endif // !WIN32
 
+static inline bool is_1byte_message(uint8_t c) {
+	return c == ' ' || (0xa1 <= c && c <= 0xdd);
+}
+
+static inline bool is_2byte_message(uint8_t c) {
+	return (0x81 <= c && c <= 0x9f) || 0xe0 <= c;
+}
+
 // resource.cpp
 SDL_RWops* open_resource(const char* name, const char* type);
 
diff --git a/src/fileio.cpp b/src/fileio.cpp
index 4bb0e6c..ed8f215 100644
--- a/src/fileio.cpp
+++ b/src/fileio.cpp
@@ -128,7 +128,7 @@ void FILEIO::Fgets(char *dest, int length)
 			if(p < length + 1) {
 				dest[p++] = c;
 			}
-			if((0x81 <= c && c <= 0x9f) || 0xe0 <= c) {
+			if(is_2byte_message(c)) {
 				if(p < length + 1) {
 					dest[p++] = Fgetc();
 				}
diff --git a/src/sys/ags_text.cpp b/src/sys/ags_text.cpp
index ca61060..29e9b10 100644
--- a/src/sys/ags_text.cpp
+++ b/src/sys/ags_text.cpp
@@ -40,7 +40,7 @@ void AGS::draw_text(const char* string)
 	while(string[p] != '\0') {
 		// 文字コード取得
 		uint16 code = (uint8)string[p++];
-		if((0x81 <= code && code <= 0x9f) || 0xe0 <= code) {
+		if(is_2byte_message(code)) {
 			code = (code << 8) | (uint8)string[p++];
 		}
 		if(draw_hankaku) {
diff --git a/src/sys/nact.cpp b/src/sys/nact.cpp
index eb66dec..107e0bf 100644
--- a/src/sys/nact.cpp
+++ b/src/sys/nact.cpp
@@ -278,8 +278,7 @@ void NACT::execute()
 			cmd_z();
 			break;
 		default:
-			if(cmd == 0x20 || (0xa1 <= cmd && cmd <= 0xdd)) {
-				// message (1 byte)
+			if(is_1byte_message(cmd)) {
 				char string[3];
 				string[0] = cmd;
 				string[1] = '\0';
@@ -313,8 +312,7 @@ void NACT::execute()
 					string[2] = '\0';
 				}
 				output_console(string);
-			} else if((0x81 <= cmd && cmd <= 0x9f) || 0xe0 <= cmd) {
-				// message (2 bytes)
+			} else if(is_2byte_message(cmd)) {
 				char string[3];
 				string[0] = cmd;
 				string[1] = getd();
diff --git a/src/sys/nact_sys1.cpp b/src/sys/nact_sys1.cpp
index 0348d9a..5df7f7f 100644
--- a/src/sys/nact_sys1.cpp
+++ b/src/sys/nact_sys1.cpp
@@ -204,9 +204,9 @@ void NACT_Sys1::cmd_branch()
 			} else if(cmd == 'Z') {
 				cali();
 				cali();
-			} else if(cmd == 0x20 || (0xa1 <= cmd && cmd <= 0xdd)) {
+			} else if(is_1byte_message(cmd)) {
 				// message (1 byte)
-			} else if((0x81 <= cmd && cmd <= 0x9f) || 0xe0 <= cmd) {
+			} else if(is_2byte_message(cmd)) {
 				// message (2 bytes)
 				getd();
 			} else {
diff --git a/src/sys/nact_sys2.cpp b/src/sys/nact_sys2.cpp
index 4802f0f..ee4a018 100644
--- a/src/sys/nact_sys2.cpp
+++ b/src/sys/nact_sys2.cpp
@@ -171,9 +171,9 @@ void NACT_Sys2::cmd_branch()
 			} else if(cmd == 'M') {
 				for(;;) {
 					uint8 val = getd();
-					if(val == 0x20 || (0xa1 <= val && val <= 0xdd)) {
+					if(is_1byte_message(val)) {
 						// message (1 byte)
-					} else if((0x81 <= val && val <= 0x9f) || 0xe0 <= val) {
+					} else if(is_2byte_message(val)) {
 						// message (2 bytes)
 						getd();
 					} else if(val == ':') {
@@ -255,9 +255,9 @@ void NACT_Sys2::cmd_branch()
 			} else if(cmd == 'Z') {
 				cali();
 				cali();
-			} else if(cmd == 0x20 || (0xa1 <= cmd && cmd <= 0xdd)) {
+			} else if(is_1byte_message(cmd)) {
 				// message (1 byte)
-			} else if((0x81 <= cmd && cmd <= 0x9f) || 0xe0 <= cmd) {
+			} else if(is_2byte_message(cmd)) {
 				// message (2 bytes)
 				getd();
 			} else {
@@ -1082,8 +1082,7 @@ void NACT_Sys2::cmd_m()
 	int d, p = 0;
 
 	while((d = getd()) != ':') {
-		if((0x81 <= d && d <= 0x9f) || 0xe0 <= d) {
-			// 2bytes
+		if(is_2byte_message(d)) {
 			string[p++] = d;
 			string[p++] = getd();
 		} else {
@@ -1474,7 +1473,7 @@ void NACT_Sys2::cmd_y()
 				int len = cmd - 220, p = 0, q = 0;
 				uint8 d;
 				while((d = (uint8)tvar[param - 1][p]) != '\0') {
-					if((0x81 <= d && d <= 0x9f) || 0xe0 <= d) {
+					if(is_2byte_message(d)) {
 						string[p] = tvar[param - 1][p];
 						p++;
 					}
diff --git a/src/sys/nact_sys3.cpp b/src/sys/nact_sys3.cpp
index 2b477a6..d4a73e4 100644
--- a/src/sys/nact_sys3.cpp
+++ b/src/sys/nact_sys3.cpp
@@ -798,8 +798,7 @@ void NACT_Sys3::cmd_m()
 	int d, p = 0;
 
 	while((d = getd()) != ':') {
-		if((0x81 <= d && d <= 0x9f) || 0xe0 <= d) {
-			// 2bytes
+		if(is_2byte_message(d)) {
 			string[p++] = d;
 			string[p++] = getd();
 		} else {
@@ -1385,7 +1384,7 @@ void NACT_Sys3::cmd_y()
 					} else {
 						uint8 code = tvar[param - 1][s++];
 						string[d++] = code;
-						if((0x81 <= code && code <= 0x9f) || 0xe0 <= code) {
+						if(is_2byte_message(code)) {
 							string[d++] = tvar[param - 1][s++];
 						}
 					}
diff --git a/src/win/nact_win.cpp b/src/win/nact_win.cpp
index 516b16a..25935dd 100644
--- a/src/win/nact_win.cpp
+++ b/src/win/nact_win.cpp
@@ -69,7 +69,7 @@ INT_PTR CALLBACK TextDialogProc(HWND hDlg, UINT msg, WPARAM wParam, LPARAM lPara
 					pnt = cnt = 0;
 					while(string[pnt] != '\0') {
 						unsigned char dat = string[pnt];
-						if((0x81 <= dat && dat <= 0x9f) || 0xe0 <= dat) {
+						if(is_2byte_message(dat)) {
 							pnt++;
 						}
 						pnt++;