diff --git a/mrbgems/mruby-bigint/README.md b/mrbgems/mruby-bigint/README.md
index bf15b7f24..81b368601 100644
--- a/mrbgems/mruby-bigint/README.md
+++ b/mrbgems/mruby-bigint/README.md
@@ -3,7 +3,3 @@
 This extension uses fgmp, which is a public domain implementation of a subset of the GNU gmp library by Mark Henderson <markh@wimsey.bc.ca>
 But it's heavily modified to fit with mruby. You can get the original source code from <https://github.com/deischi/fgmp.git>
 You can read the original README for fgmp in <README-fgmp.md>
-
-## Implementation
-
-This mrbgem is only to turn on `MRB_USE_BIGINT` flag. The implementation resides in `include/mruby/bigint.h` and `src/bigint.c`.
diff --git a/src/bigint.c b/mrbgems/mruby-bigint/core/bigint.c
similarity index 99%
rename from src/bigint.c
rename to mrbgems/mruby-bigint/core/bigint.c
index 66683f5e1..93ca97549 100644
--- a/src/bigint.c
+++ b/mrbgems/mruby-bigint/core/bigint.c
@@ -3,16 +3,15 @@
 **
 ** See Copyright Notice in mruby.h
 */
-#ifdef MRB_USE_BIGINT
 
 #include <mruby.h>
 #include <mruby/object.h>
 #include <mruby/numeric.h>
-#include <mruby/bigint.h>
 #include <mruby/array.h>
 #include <mruby/string.h>
 #include <mruby/internal.h>
 #include <string.h>
+#include "bigint.h"
 
 static void
 mpz_init(mrb_state *mrb, mpz_t *s)
@@ -1458,4 +1457,3 @@ mrb_bint_rshift(mrb_state *mrb, mrb_value x, mrb_int width)
   }
   return bint_norm(mrb, b2);
 }
-#endif  /* MRB_USE_BIGINT */
diff --git a/include/mruby/bigint.h b/mrbgems/mruby-bigint/core/bigint.h
similarity index 97%
rename from include/mruby/bigint.h
rename to mrbgems/mruby-bigint/core/bigint.h
index 925a5d27a..24ed6ee7c 100644
--- a/include/mruby/bigint.h
+++ b/mrbgems/mruby-bigint/core/bigint.h
@@ -3,7 +3,6 @@
 **
 ** See Copyright Notice in mruby.h
 */
-#ifdef MRB_USE_BIGINT
 
 #ifndef MRUBY_BIGINT_H
 #define MRUBY_BIGINT_H
@@ -85,6 +84,4 @@ struct RBigint {
 
 #define RBIGINT(v) ((struct RBigint*)mrb_ptr(v))
 
-#endif
-
-#endif  /* MRB_USE_BIGINT */
+#endif  /* MRUBY_BIGINT_H */
diff --git a/mrbgems/mruby-bigint/mrbgem.rake b/mrbgems/mruby-bigint/mrbgem.rake
index 528a7ee70..a1039bf57 100644
--- a/mrbgems/mruby-bigint/mrbgem.rake
+++ b/mrbgems/mruby-bigint/mrbgem.rake
@@ -3,4 +3,8 @@ MRuby::Gem::Specification.new('mruby-bigint') do |spec|
   spec.author  = 'mruby developers'
   spec.summary = 'Integer class extension to multiple-precision'
   spec.build.defines << "MRB_USE_BIGINT"
+
+  spec.build.libmruby_core_objs << Dir.glob(File.join(__dir__, "core/**/*.c")).map { |fn|
+    objfile(fn.relative_path_from(__dir__).pathmap("#{spec.build_dir}/%X"))
+  }
 end
diff --git a/mrbgems/mruby-numeric-ext/src/numeric_ext.c b/mrbgems/mruby-numeric-ext/src/numeric_ext.c
index 1010d8fa7..7ea0e9772 100644
--- a/mrbgems/mruby-numeric-ext/src/numeric_ext.c
+++ b/mrbgems/mruby-numeric-ext/src/numeric_ext.c
@@ -4,8 +4,6 @@
 #include <mruby/presym.h>
 
 #ifdef MRB_USE_BIGINT
-#include <mruby/bigint.h>
-
 static mrb_value
 bint_allbits(mrb_state *mrb, mrb_value x, mrb_value y)
 {