diff --git a/include/picrin/pair.h b/include/picrin/pair.h
index 49e3f73f..d6cf779d 100644
--- a/include/picrin/pair.h
+++ b/include/picrin/pair.h
@@ -57,7 +57,7 @@ pic_value pic_list5(pic_state *, pic_value, pic_value, pic_value, pic_value, pic
 pic_value pic_list6(pic_state *, pic_value, pic_value, pic_value, pic_value, pic_value, pic_value);
 pic_value pic_list7(pic_state *, pic_value, pic_value, pic_value, pic_value, pic_value, pic_value, pic_value);
 pic_value pic_list_by_array(pic_state *, int, pic_value *);
-pic_value pic_make_list(pic_state *, int, pic_value);
+pic_value pic_make_list(pic_state *, size_t, pic_value);
 
 #define pic_for_each(var, list)                                 \
   pic_for_each_helper_(var, GENSYM(tmp), list)
@@ -69,7 +69,7 @@ pic_value pic_make_list(pic_state *, int, pic_value);
 #define pic_push(pic, item, place) (place = pic_cons(pic, item, place))
 #define pic_pop(pic, place) (place = pic_cdr(pic, place))
 
-int pic_length(pic_state *, pic_value);
+size_t pic_length(pic_state *, pic_value);
 pic_value pic_reverse(pic_state *, pic_value);
 pic_value pic_append(pic_state *, pic_value, pic_value);
 
@@ -88,9 +88,9 @@ pic_value pic_cadr(pic_state *, pic_value);
 pic_value pic_cdar(pic_state *, pic_value);
 pic_value pic_cddr(pic_state *, pic_value);
 
-pic_value pic_list_tail(pic_state *, pic_value, int);
-pic_value pic_list_ref(pic_state *, pic_value, int);
-void pic_list_set(pic_state *, pic_value, int, pic_value);
+pic_value pic_list_tail(pic_state *, pic_value, size_t);
+pic_value pic_list_ref(pic_state *, pic_value, size_t);
+void pic_list_set(pic_state *, pic_value, size_t, pic_value);
 pic_value pic_list_copy(pic_state *, pic_value);
 
 #if defined(__cplusplus)
diff --git a/pair.c b/pair.c
index 6a9687d8..c2a775dc 100644
--- a/pair.c
+++ b/pair.c
@@ -172,10 +172,10 @@ pic_list_by_array(pic_state *pic, int c, pic_value *vs)
 }
 
 pic_value
-pic_make_list(pic_state *pic, int k, pic_value fill)
+pic_make_list(pic_state *pic, size_t k, pic_value fill)
 {
   pic_value list;
-  int i;
+  size_t i;
 
   list = pic_nil_value();
   for (i = 0; i < k; ++i) {
@@ -185,10 +185,10 @@ pic_make_list(pic_state *pic, int k, pic_value fill)
   return list;
 }
 
-int
+size_t
 pic_length(pic_state *pic, pic_value obj)
 {
-  int c = 0;
+  size_t c = 0;
 
   if (! pic_list_p(obj)) {
     pic_errorf(pic, "length: expected list, but got ~s", obj);
@@ -375,7 +375,7 @@ pic_cddr(pic_state *pic, pic_value v)
 }
 
 pic_value
-pic_list_tail(pic_state *pic, pic_value list, int i)
+pic_list_tail(pic_state *pic, pic_value list, size_t i)
 {
   while (i-- > 0) {
     list = pic_cdr(pic, list);
@@ -384,13 +384,13 @@ pic_list_tail(pic_state *pic, pic_value list, int i)
 }
 
 pic_value
-pic_list_ref(pic_state *pic, pic_value list, int i)
+pic_list_ref(pic_state *pic, pic_value list, size_t i)
 {
   return pic_car(pic, pic_list_tail(pic, list, i));
 }
 
 void
-pic_list_set(pic_state *pic, pic_value list, int i, pic_value obj)
+pic_list_set(pic_state *pic, pic_value list, size_t i, pic_value obj)
 {
   pic_pair_ptr(pic_list_tail(pic, list, i))->car = obj;
 }