diff --git a/parse.c b/parse.c
index fef119219b3cef25f594fc71aaa5e1ff2e499367..fb75e2647b8958fcdd06712ea34ab2fe42c2dccb 100644
--- a/parse.c
+++ b/parse.c
@@ -2153,6 +2153,10 @@ static Node *funcall(Token **rest, Token *tok) {
         error_tok(arg->tok, "passing struct or union is not supported yet");
       arg = new_cast(arg, param_ty);
       param_ty = param_ty->next;
+    } else if (arg->ty->kind == TY_FLOAT) {
+      // If parameter type is omitted (e.g. in "..."), float
+      // arguments are promoted to double.
+      arg = new_cast(arg, ty_double);
     }
 
     cur = cur->next = arg;
diff --git a/test/function.c b/test/function.c
index df271a2ab3c7ddbfbd7a8f98c7d4ab1c2f670ec6..f4494b5d899003c815831b487fff179979dea5ab 100644
--- a/test/function.c
+++ b/test/function.c
@@ -174,6 +174,8 @@ int main() {
   ASSERT(7, add_float3(2.5, 2.5, 2.5));
   ASSERT(7, add_double3(2.5, 2.5, 2.5));
 
+  ASSERT(0, ({ char buf[100]; sprintf(buf, "%.1f", (float)3.5); strcmp(buf, "3.5"); }));
+
   printf("OK\n");
   return 0;
 }