diff --git a/mrbgems/mruby-metaprog/src/metaprog.c b/mrbgems/mruby-metaprog/src/metaprog.c
index 4337db48b..3b93574c1 100644
--- a/mrbgems/mruby-metaprog/src/metaprog.c
+++ b/mrbgems/mruby-metaprog/src/metaprog.c
@@ -294,7 +294,6 @@ mrb_obj_public_methods(mrb_state *mrb, mrb_value self)
 static mrb_value
 mrb_obj_singleton_methods(mrb_state *mrb, mrb_bool recur, mrb_value obj)
 {
-  khint_t i;
   mrb_value ary;
   struct RClass *klass;
   khash_t(st) *set = kh_init(st, mrb);
@@ -314,7 +313,7 @@ mrb_obj_singleton_methods(mrb_state *mrb, mrb_bool recur, mrb_value obj)
   }
 
   ary = mrb_ary_new(mrb);
-  for (i=0;i<kh_end(set);i++) {
+  for (khint_t i=0;i<kh_end(set);i++) {
     if (kh_exist(set, i)) {
       mrb_ary_push(mrb, ary, mrb_symbol_value(kh_key(set, i)));
     }
diff --git a/src/array.c b/src/array.c
index f9d747ef9..a6a41e809 100644
--- a/src/array.c
+++ b/src/array.c
@@ -90,9 +90,7 @@ mrb_ary_new(mrb_state *mrb)
 static inline void
 array_copy(mrb_value *dst, const mrb_value *src, mrb_int size)
 {
-  mrb_int i;
-
-  for (i = 0; i < size; i++) {
+  for (mrb_int i = 0; i < size; i++) {
     dst[i] = src[i];
   }
 }
@@ -1134,9 +1132,8 @@ static mrb_value
 mrb_ary_index_m(mrb_state *mrb, mrb_value self)
 {
   mrb_value obj = mrb_get_arg1(mrb);
-  mrb_int i;
 
-  for (i = 0; i < RARRAY_LEN(self); i++) {
+  for (mrb_int i = 0; i < RARRAY_LEN(self); i++) {
     if (mrb_equal(mrb, RARRAY_PTR(self)[i], obj)) {
       return mrb_int_value(mrb, i);
     }
@@ -1148,9 +1145,10 @@ static mrb_value
 mrb_ary_rindex_m(mrb_state *mrb, mrb_value self)
 {
   mrb_value obj = mrb_get_arg1(mrb);
-  mrb_int i, len;
 
-  for (i = RARRAY_LEN(self) - 1; i >= 0; i--) {
+  for (mrb_int i = RARRAY_LEN(self) - 1; i >= 0; i--) {
+    mrb_int len;
+
     if (mrb_equal(mrb, RARRAY_PTR(self)[i], obj)) {
       return mrb_int_value(mrb, i);
     }
@@ -1248,11 +1246,10 @@ mrb_ary_entry(mrb_value ary, mrb_int n)
 static mrb_value
 join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)
 {
-  mrb_int i;
   mrb_value result, val, tmp;
 
   /* check recursive */
-  for (i=0; i<RARRAY_LEN(list); i++) {
+  for (mrb_int i=0; i<RARRAY_LEN(list); i++) {
     if (mrb_obj_equal(mrb, ary, RARRAY_PTR(list)[i])) {
       mrb_raise(mrb, E_ARGUMENT_ERROR, "recursive array join");
     }
@@ -1262,7 +1259,7 @@ join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)
 
   result = mrb_str_new_capa(mrb, 64);
 
-  for (i=0; i<RARRAY_LEN(ary); i++) {
+  for (mrb_int i=0; i<RARRAY_LEN(ary); i++) {
     if (i > 0 && !mrb_nil_p(sep)) {
       mrb_str_cat_str(mrb, result, sep);
     }
diff --git a/src/codedump.c b/src/codedump.c
index b4633e365..c7b81971b 100644
--- a/src/codedump.c
+++ b/src/codedump.c
@@ -99,10 +99,9 @@ codedump(mrb_state *mrb, const mrb_irep *irep, FILE *out)
           irep->nregs, irep->nlocals, (int)irep->plen, (int)irep->slen, (int)irep->rlen, (int)irep->ilen);
 
   if (irep->lv) {
-    int i;
     int head = FALSE;
 
-    for (i = 1; i < irep->nlocals; i++) {
+    for (int i = 1; i < irep->nlocals; i++) {
       char const *s = mrb_sym_dump(mrb, irep->lv[i - 1]);
       if (s) {
         if (!head) {
@@ -115,10 +114,9 @@ codedump(mrb_state *mrb, const mrb_irep *irep, FILE *out)
   }
 
   if (irep->clen > 0) {
-    int i = irep->clen;
     const struct mrb_irep_catch_handler *e = mrb_irep_catch_handler_table(irep);
 
-    for (; i > 0; i--,e++) {
+    for (int i = irep->clen; i > 0; i--,e++) {
       uint32_t begin = mrb_irep_catch_handler_unpack(e->begin);
       uint32_t end = mrb_irep_catch_handler_unpack(e->end);
       uint32_t target = mrb_irep_catch_handler_unpack(e->target);
@@ -620,11 +618,9 @@ codedump(mrb_state *mrb, const mrb_irep *irep, FILE *out)
 static void
 codedump_recur(mrb_state *mrb, const mrb_irep *irep, FILE *out)
 {
-  int i;
-
   codedump(mrb, irep, out);
   if (irep->reps) {
-    for (i=0; i<irep->rlen; i++) {
+    for (int i=0; i<irep->rlen; i++) {
       codedump_recur(mrb, irep->reps[i], out);
     }
   }
diff --git a/src/gc.c b/src/gc.c
index eb1dee94c..66626d20c 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -426,7 +426,6 @@ mrb_gc_unregister(mrb_state *mrb, mrb_value obj)
   mrb_sym root;
   mrb_value table;
   struct RArray *a;
-  mrb_int i;
 
   if (mrb_immediate_p(obj)) return;
   root = GC_ROOT_SYM;
@@ -438,7 +437,7 @@ mrb_gc_unregister(mrb_state *mrb, mrb_value obj)
   }
   a = mrb_ary_ptr(table);
   mrb_ary_modify(mrb, a);
-  for (i = 0; i < ARY_LEN(a); i++) {
+  for (mrb_int i = 0; i < ARY_LEN(a); i++) {
     if (mrb_ptr(ARY_PTR(a)[i]) == mrb_ptr(obj)) {
       mrb_int len = ARY_LEN(a)-1;
       mrb_value *ptr = ARY_PTR(a);
@@ -529,8 +528,7 @@ add_gray_list(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
 static void
 mark_context_stack(mrb_state *mrb, struct mrb_context *c)
 {
-  size_t i;
-  size_t e;
+  size_t i, e;
   mrb_value nil;
 
   if (c->stbase == NULL) return;
@@ -626,13 +624,12 @@ gc_mark_children(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
   case MRB_TT_ENV:
     {
       struct REnv *e = (struct REnv*)obj;
-      mrb_int i, len;
 
       if (MRB_ENV_ONSTACK_P(e) && e->cxt && e->cxt->fib) {
         mrb_gc_mark(mrb, (struct RBasic*)e->cxt->fib);
       }
-      len = MRB_ENV_LEN(e);
-      for (i=0; i<len; i++) {
+      mrb_int len = MRB_ENV_LEN(e);
+      for (mrb_int i=0; i<len; i++) {
         mrb_gc_mark_value(mrb, e->stack[i]);
       }
     }
@@ -1515,10 +1512,9 @@ gc_each_objects(mrb_state *mrb, mrb_gc *gc, mrb_each_object_callback *callback,
   page = gc->heaps;
   while (page != NULL) {
     RVALUE *p;
-    int i;
 
     p = objects(page);
-    for (i=0; i < MRB_HEAP_PAGE_SIZE; i++) {
+    for (int i=0; i < MRB_HEAP_PAGE_SIZE; i++) {
       if ((*callback)(mrb, &p[i].as.basic, data) == MRB_EACH_OBJ_BREAK)
         return;
     }
diff --git a/src/string.c b/src/string.c
index d4277b2b9..d3c3b2e17 100644
--- a/src/string.c
+++ b/src/string.c
@@ -467,11 +467,10 @@ mrb_memsearch_qs(const unsigned char *xs, mrb_int m, const unsigned char *ys, mr
   else {
     const unsigned char *x = xs, *xe = xs + m;
     const unsigned char *y = ys;
-    int i;
     ptrdiff_t qstable[256];
 
     /* Preprocessing */
-    for (i = 0; i < 256; i++)
+    for (int i = 0; i < 256; i++)
       qstable[i] = m + 1;
     for (; x < xe; x++)
       qstable[*x] = xe - x;
diff --git a/src/symbol.c b/src/symbol.c
index 4c1ce69ba..5953099c7 100644
--- a/src/symbol.c
+++ b/src/symbol.c
@@ -82,12 +82,11 @@ sym_inline_pack(const char *name, size_t len)
 
   char c;
   const char *p;
-  size_t i;
   mrb_sym sym = 0;
 
   if (len > pack_length_max) return 0; /* too long */
   if (len == 0) return 0; /* empty string */
-  for (i=0; i<len; i++) {
+  for (size_t i=0; i<len; i++) {
     uint32_t bits;
 
     c = name[i];
@@ -352,7 +351,7 @@ mrb_free_symtbl(mrb_state *mrb)
 {
   mrb_sym i, lim;
 
-  for (i=1, lim=mrb->symidx+1; i<lim; i++) {
+  for (i=1,lim=mrb->symidx+1; i<lim; i++) {
     if (!sym_lit_p(mrb, i)) {
       mrb_free(mrb, (char*)mrb->symtbl[i]);
     }
diff --git a/src/variable.c b/src/variable.c
index 09d872b07..fef9f29c4 100644
--- a/src/variable.c
+++ b/src/variable.c
@@ -175,15 +175,13 @@ iv_del(mrb_state *mrb, iv_tbl *t, mrb_sym sym, mrb_value *vp)
 static void
 iv_foreach(mrb_state *mrb, iv_tbl *t, mrb_iv_foreach_func *func, void *p)
 {
-  int i;
-
   if (t == NULL) return;
   if (t->alloc == 0) return;
   if (t->size == 0) return;
 
   mrb_sym *keys = (mrb_sym*)&t->ptr[t->alloc];
   mrb_value *vals = t->ptr;
-  for (i=0; i<t->alloc; i++) {
+  for (int i=0; i<t->alloc; i++) {
     if (IV_KEY_P(keys[i])) {
       if ((*func)(mrb, keys[i], vals[i], p) != 0) {
         return;
@@ -207,7 +205,6 @@ static iv_tbl*
 iv_copy(mrb_state *mrb, iv_tbl *t)
 {
   iv_tbl *t2;
-  int i;
 
   if (t == NULL) return NULL;
   if (t->alloc == 0) return NULL;
@@ -216,7 +213,7 @@ iv_copy(mrb_state *mrb, iv_tbl *t)
   mrb_sym *keys = (mrb_sym*)&t->ptr[t->alloc];
   mrb_value *vals = t->ptr;
   t2 = iv_new(mrb);
-  for (i=0; i<t->alloc; i++) {
+  for (int i=0; i<t->alloc; i++) {
     if (IV_KEY_P(keys[i])) {
       iv_put(mrb, t2, keys[i], vals[i]);
     }
@@ -1135,9 +1132,7 @@ mrb_obj_iv_tbl_memsize(mrb_value obj)
 mrb_bool
 mrb_ident_p(const char *s, mrb_int len)
 {
-  mrb_int i;
-
-  for (i = 0; i < len; i++) {
+  for (mrb_int i = 0; i < len; i++) {
     if (!identchar(s[i])) return FALSE;
   }
   return TRUE;
diff --git a/src/vm.c b/src/vm.c
index 732cbe2a8..9887a07d5 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -2853,10 +2853,9 @@ RETRY_TRY_BLOCK:
 
     CASE(OP_HASH, BB) {
       mrb_value hash = mrb_hash_new_capa(mrb, b);
-      int i;
       int lim = a+b*2;
 
-      for (i=a; i<lim; i+=2) {
+      for (int i=a; i<lim; i+=2) {
         mrb_hash_set(mrb, hash, regs[i], regs[i+1]);
       }
       regs[a] = hash;
@@ -2866,12 +2865,11 @@ RETRY_TRY_BLOCK:
 
     CASE(OP_HASHADD, BB) {
       mrb_value hash;
-      int i;
       int lim = a+b*2+1;
 
       hash = regs[a];
       mrb_ensure_hash_type(mrb, hash);
-      for (i=a+1; i<lim; i+=2) {
+      for (int i=a+1; i<lim; i+=2) {
         mrb_hash_set(mrb, hash, regs[i], regs[i+1]);
       }
       mrb_gc_arena_restore(mrb, ai);