diff --git a/src/Limonp/StringUtil.hpp b/src/Limonp/StringUtil.hpp
index e6cc15d..ed8fdbc 100644
--- a/src/Limonp/StringUtil.hpp
+++ b/src/Limonp/StringUtil.hpp
@@ -204,7 +204,7 @@ namespace Limonp
                 vec.push_back(str[i]);
                 i++;
             }
-            else if ((unsigned char)str[i] <= 0xdf && i + 1 < len) // 110xxxxxx
+            else if ((uint8_t)str[i] <= 0xdf && i + 1 < len) // 110xxxxxx
             {
                 ch1 = (str[i] >> 2) & 0x07;
                 ch2 = (str[i+1] & 0x3f) | ((str[i] & 0x03) << 6 );
@@ -212,10 +212,10 @@ namespace Limonp
                 vec.push_back(tmp);
                 i += 2;
             }
-            else if((unsigned char)str[i] <= 0xef && i + 2 < len)
+            else if((uint8_t)str[i] <= 0xef && i + 2 < len)
             {
-                ch1 = (str[i] << 4) | ((str[i+1] >> 2) & 0x0f );
-                ch2 = ((str[i+1]<<6) & 0xc0) | (str[i+2] & 0x3f); 
+                ch1 = ((uint8_t)str[i] << 4) | ((str[i+1] >> 2) & 0x0f );
+                ch2 = (((uint8_t)str[i+1]<<6) & 0xc0) | (str[i+2] & 0x3f); 
                 tmp = (((uint16_t(ch1) & 0x00ff ) << 8) | (uint16_t(ch2) & 0x00ff));
                 vec.push_back(tmp);
                 i += 3;