diff --git a/src/dump.c b/src/dump.c
index 243278103..25908260b 100644
--- a/src/dump.c
+++ b/src/dump.c
@@ -103,7 +103,7 @@ dump_float(mrb_state *mrb, uint8_t *buf, mrb_float f)
     memcpy(buf, u.s, sizeof(double));
   }
   else {
-    int i;
+    size_t i;
 
     for (i=0; i<sizeof(double); i++) {
       buf[i] = u.s[sizeof(double)-i-1];
diff --git a/src/load.c b/src/load.c
index 392a38456..aae1e071b 100644
--- a/src/load.c
+++ b/src/load.c
@@ -55,7 +55,7 @@ str_to_double(mrb_state *mrb, const char *p, size_t len)
     memcpy(u.s, p, sizeof(double));
   }
   else {
-    int i;
+    size_t i;
     for (i=0; i<sizeof(double); i++) {
       u.s[i] = p[sizeof(double)-i-1];
     }