diff --git a/include/mruby.h b/include/mruby.h
index f18fd472b..bca53003f 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -1180,13 +1180,13 @@ MRB_API void mrb_free(mrb_state*, void*);
  */
 #define MRB_OBJ_ALLOC(mrb, tt, klass) ((MRB_VTYPE_TYPEOF(tt)*)mrb_obj_alloc(mrb, tt, klass))
 
-MRB_API mrb_value mrb_str_new(mrb_state *mrb, const char *p, size_t len);
+MRB_API mrb_value mrb_str_new(mrb_state *mrb, const char *p, mrb_int len);
 
 /**
  * Turns a C string into a Ruby string value.
  */
 MRB_API mrb_value mrb_str_new_cstr(mrb_state*, const char*);
-MRB_API mrb_value mrb_str_new_static(mrb_state *mrb, const char *p, size_t len);
+MRB_API mrb_value mrb_str_new_static(mrb_state *mrb, const char *p, mrb_int len);
 #define mrb_str_new_lit(mrb, lit) mrb_str_new_static(mrb, (lit), mrb_strlen_lit(lit))
 
 MRB_API mrb_value mrb_obj_freeze(mrb_state*, mrb_value);
diff --git a/include/mruby/string.h b/include/mruby/string.h
index b296832af..3a8d73a70 100644
--- a/include/mruby/string.h
+++ b/include/mruby/string.h
@@ -324,7 +324,7 @@ MRB_API mrb_value mrb_str_resize(mrb_state *mrb, mrb_value str, mrb_int len);
  */
 MRB_API mrb_value mrb_str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len);
 
-MRB_API mrb_value mrb_str_new_capa(mrb_state *mrb, size_t capa);
+MRB_API mrb_value mrb_str_new_capa(mrb_state *mrb, mrb_int capa);
 #define mrb_str_buf_new(mrb, capa) mrb_str_new_capa(mrb, (capa))
 
 /* NULL terminated C string from mrb_value */
diff --git a/mrbgems/mruby-compiler/core/codegen.c b/mrbgems/mruby-compiler/core/codegen.c
index 4b9ff64df..4d4d6abb7 100644
--- a/mrbgems/mruby-compiler/core/codegen.c
+++ b/mrbgems/mruby-compiler/core/codegen.c
@@ -3344,7 +3344,7 @@ codegen(codegen_scope *s, node *tree, int val)
   case NODE_STR:
     if (val) {
       char *p = (char*)tree->car;
-      size_t len = (intptr_t)tree->cdr;
+      mrb_int len = nint(tree->cdr);
       int ai = mrb_gc_arena_save(s->mrb);
       int off = new_lit(s, mrb_str_new(s->mrb, p, len));
 
@@ -3429,7 +3429,7 @@ codegen(codegen_scope *s, node *tree, int val)
   case NODE_XSTR:
     {
       char *p = (char*)tree->car;
-      size_t len = (intptr_t)tree->cdr;
+      mrb_int len = nint(tree->cdr);
       int ai = mrb_gc_arena_save(s->mrb);
       int off = new_lit(s, mrb_str_new(s->mrb, p, len));
       int sym;
diff --git a/mrbgems/mruby-compiler/core/parse.y b/mrbgems/mruby-compiler/core/parse.y
index dce985182..3496b36d0 100644
--- a/mrbgems/mruby-compiler/core/parse.y
+++ b/mrbgems/mruby-compiler/core/parse.y
@@ -6933,7 +6933,7 @@ mrb_load_detect_file_cxt(mrb_state *mrb, FILE *fp, mrbc_context *c)
     return mrb_load_exec(mrb, mrb_parse_file_continue(mrb, fp, leading.b, bufsize, c), c);
   }
   else {
-    size_t binsize;
+    mrb_int binsize;
     uint8_t *bin;
     mrb_value bin_obj = mrb_nil_value(); /* temporary string object */
     mrb_value result;
diff --git a/mrbgems/mruby-compiler/core/y.tab.c b/mrbgems/mruby-compiler/core/y.tab.c
index 2343f7c37..429c2209b 100644
--- a/mrbgems/mruby-compiler/core/y.tab.c
+++ b/mrbgems/mruby-compiler/core/y.tab.c
@@ -13164,7 +13164,7 @@ mrb_load_detect_file_cxt(mrb_state *mrb, FILE *fp, mrbc_context *c)
     return mrb_load_exec(mrb, mrb_parse_file_continue(mrb, fp, leading.b, bufsize, c), c);
   }
   else {
-    size_t binsize;
+    mrb_int binsize;
     uint8_t *bin;
     mrb_value bin_obj = mrb_nil_value(); /* temporary string object */
     mrb_value result;
diff --git a/mrbgems/mruby-socket/src/socket.c b/mrbgems/mruby-socket/src/socket.c
index fd5967cf9..a002a85a0 100644
--- a/mrbgems/mruby-socket/src/socket.c
+++ b/mrbgems/mruby-socket/src/socket.c
@@ -626,7 +626,7 @@ mrb_socket_accept2(mrb_state *mrb, mrb_value klass)
 
   mrb_get_args(mrb, "i", &s0);
   socklen = sizeof(struct sockaddr_storage);
-  sastr = mrb_str_new_capa(mrb, socklen);
+  sastr = mrb_str_new_capa(mrb, (mrb_int)socklen);
   s1 = (int)accept(s0, (struct sockaddr *)RSTRING_PTR(sastr), &socklen);
   if (s1 == -1) {
     mrb_sys_fail(mrb, "accept");
diff --git a/src/string.c b/src/string.c
index 890300432..60ad00b6f 100644
--- a/src/string.c
+++ b/src/string.c
@@ -19,7 +19,7 @@
 
 typedef struct mrb_shared_string {
   int refcnt;
-  mrb_ssize capa;
+  mrb_int capa;
   char *ptr;
 } mrb_shared_string;
 
@@ -29,26 +29,26 @@ const char mrb_digitmap[] = "0123456789abcdefghijklmnopqrstuvwxyz";
 
 static struct RString*
 str_init_normal_capa(mrb_state *mrb, struct RString *s,
-                     const char *p, size_t len, size_t capa)
+                     const char *p, mrb_int len, mrb_int capa)
 {
   char *dst = (char *)mrb_malloc(mrb, capa + 1);
   if (p) memcpy(dst, p, len);
   dst[len] = '\0';
   s->as.heap.ptr = dst;
-  s->as.heap.len = (mrb_ssize)len;
-  s->as.heap.aux.capa = (mrb_ssize)capa;
+  s->as.heap.len = len;
+  s->as.heap.aux.capa = capa;
   RSTR_UNSET_TYPE_FLAG(s);
   return s;
 }
 
 static struct RString*
-str_init_normal(mrb_state *mrb, struct RString *s, const char *p, size_t len)
+str_init_normal(mrb_state *mrb, struct RString *s, const char *p, mrb_int len)
 {
   return str_init_normal_capa(mrb, s, p, len, len);
 }
 
 static struct RString*
-str_init_embed(struct RString *s, const char *p, size_t len)
+str_init_embed(struct RString *s, const char *p, mrb_int len)
 {
   if (p) memcpy(RSTR_EMBED_PTR(s), p, len);
   RSTR_EMBED_PTR(s)[len] = '\0';
@@ -58,10 +58,10 @@ str_init_embed(struct RString *s, const char *p, size_t len)
 }
 
 static struct RString*
-str_init_nofree(struct RString *s, const char *p, size_t len)
+str_init_nofree(struct RString *s, const char *p, mrb_int len)
 {
   s->as.heap.ptr = (char *)p;
-  s->as.heap.len = (mrb_ssize)len;
+  s->as.heap.len = len;
   s->as.heap.aux.capa = 0;             /* nofree */
   RSTR_SET_TYPE_FLAG(s, NOFREE);
   return s;
@@ -97,7 +97,7 @@ str_init_fshared(const struct RString *orig, struct RString *s, struct RString *
 }
 
 static struct RString*
-str_init_modifiable(mrb_state *mrb, struct RString *s, const char *p, size_t len)
+str_init_modifiable(mrb_state *mrb, struct RString *s, const char *p, mrb_int len)
 {
   if (RSTR_EMBEDDABLE_P(len)) {
     return str_init_embed(s, p, len);
@@ -108,7 +108,7 @@ str_init_modifiable(mrb_state *mrb, struct RString *s, const char *p, size_t len
 }
 
 static struct RString*
-str_new_static(mrb_state *mrb, const char *p, size_t len)
+str_new_static(mrb_state *mrb, const char *p, mrb_int len)
 {
   if (RSTR_EMBEDDABLE_P(len)) {
     return str_init_embed(mrb_obj_alloc_string(mrb), p, len);
@@ -120,7 +120,7 @@ str_new_static(mrb_state *mrb, const char *p, size_t len)
 }
 
 static struct RString*
-str_new(mrb_state *mrb, const char *p, size_t len)
+str_new(mrb_state *mrb, const char *p, mrb_int len)
 {
   if (RSTR_EMBEDDABLE_P(len)) {
     return str_init_embed(mrb_obj_alloc_string(mrb), p, len);
@@ -135,7 +135,7 @@ str_new(mrb_state *mrb, const char *p, size_t len)
 }
 
 MRB_API mrb_value
-mrb_str_new_capa(mrb_state *mrb, size_t capa)
+mrb_str_new_capa(mrb_state *mrb, mrb_int capa)
 {
   struct RString *s;
 
@@ -155,7 +155,7 @@ mrb_str_new_capa(mrb_state *mrb, size_t capa)
 }
 
 static void
-resize_capa(mrb_state *mrb, struct RString *s, size_t capacity)
+resize_capa(mrb_state *mrb, struct RString *s, mrb_int capacity)
 {
 #if SIZE_MAX > MRB_SSIZE_MAX
     mrb_assert(capacity < MRB_SSIZE_MAX);
@@ -172,7 +172,7 @@ resize_capa(mrb_state *mrb, struct RString *s, size_t capacity)
 }
 
 MRB_API mrb_value
-mrb_str_new(mrb_state *mrb, const char *p, size_t len)
+mrb_str_new(mrb_state *mrb, const char *p, mrb_int len)
 {
   return mrb_obj_value(str_new(mrb, p, len));
 }
@@ -181,7 +181,7 @@ MRB_API mrb_value
 mrb_str_new_cstr(mrb_state *mrb, const char *p)
 {
   struct RString *s;
-  size_t len;
+  mrb_int len;
 
   if (p) {
     len = strlen(p);
@@ -196,7 +196,7 @@ mrb_str_new_cstr(mrb_state *mrb, const char *p)
 }
 
 MRB_API mrb_value
-mrb_str_new_static(mrb_state *mrb, const char *p, size_t len)
+mrb_str_new_static(mrb_state *mrb, const char *p, mrb_int len)
 {
   struct RString *s = str_new_static(mrb, p, len);
   return mrb_obj_value(s);
@@ -225,12 +225,12 @@ str_modify_keep_ascii(mrb_state *mrb, struct RString *s)
       mrb_free(mrb, shared);
     }
     else {
-      str_init_modifiable(mrb, s, s->as.heap.ptr, (size_t)s->as.heap.len);
+      str_init_modifiable(mrb, s, s->as.heap.ptr, s->as.heap.len);
       str_decref(mrb, shared);
     }
   }
   else if (RSTR_NOFREE_P(s) || RSTR_FSHARED_P(s)) {
-    str_init_modifiable(mrb, s, s->as.heap.ptr, (size_t)s->as.heap.len);
+    str_init_modifiable(mrb, s, s->as.heap.ptr, s->as.heap.len);
   }
 }
 
@@ -833,7 +833,7 @@ mrb_str_to_cstr(mrb_state *mrb, mrb_value str0)
   struct RString *s;
 
   const char *p = RSTRING_PTR(str0);
-  size_t len = RSTRING_LEN(str0);
+  mrb_int len = RSTRING_LEN(str0);
   check_null_byte(mrb, str0);
   if (RSTR_EMBEDDABLE_P(len)) {
     s = str_init_embed(mrb_obj_alloc_string(mrb), p, len);
@@ -920,7 +920,7 @@ mrb_str_bytesize(mrb_state *mrb, mrb_value self)
 static mrb_value
 mrb_str_times(mrb_state *mrb, mrb_value self)
 {
-  mrb_int n,len,times;
+  mrb_int n, len, times;
   struct RString *str2;
   char *p;
 
@@ -2644,7 +2644,7 @@ MRB_API mrb_value
 mrb_str_cat(mrb_state *mrb, mrb_value str, const char *ptr, size_t len)
 {
   struct RString *s = mrb_str_ptr(str);
-  size_t capa;
+  mrb_int capa;
   size_t total;
   ptrdiff_t off = -1;
 
diff --git a/src/vm.c b/src/vm.c
index aa5569503..09f524a9d 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -2729,7 +2729,7 @@ RETRY_TRY_BLOCK:
     }
 
     CASE(OP_STRING, BB) {
-      size_t len;
+      mrb_int len;
 
       mrb_assert((pool[b].tt&IREP_TT_NFLAG)==0);
       len = pool[b].tt >> 2;