diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 79b4743ac..1209228c3 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1339,19 +1339,9 @@ str_valid_enc_p(mrb_state *mrb, mrb_value str)
   const char *p = RSTR_PTR(s);
   const char *e = p + byte_len;
   while (p < e) {
-    mrb_int len = mrb_utf8len_table[(unsigned char)p[0] >> 3];
-    if (len == 0 || len > e - p)
-      return mrb_false_value();
-    switch (len) {
-    case 4:
-      if (utf8_islead(p[3])) return mrb_false_value();
-    case 3:
-      if (utf8_islead(p[2])) return mrb_false_value();
-    case 2:
-      if (utf8_islead(p[1])) return mrb_false_value();
-    default:
-      break;
-    }
+    mrb_int len = mrb_utf8len(p, e);
+
+    if (len == 1 && (*p & 0x80)) return mrb_false_value();
     p += len;
     utf8_len++;
   }