diff --git a/mrbgems/mruby-cmath/mrbgem.rake b/mrbgems/mruby-cmath/mrbgem.rake
index 00ac4b091..8a4d5ab59 100644
--- a/mrbgems/mruby-cmath/mrbgem.rake
+++ b/mrbgems/mruby-cmath/mrbgem.rake
@@ -4,5 +4,5 @@ MRuby::Gem::Specification.new('mruby-cmath') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
   spec.summary = 'standard Math module with complex'
-  spec.add_dependency 'mruby-complex'
+  spec.add_dependency 'mruby-complex', :core => 'mruby-complex'
 end
diff --git a/mrbgems/mruby-os-memsize/mrbgem.rake b/mrbgems/mruby-os-memsize/mrbgem.rake
index b5c163bba..8bd04d951 100644
--- a/mrbgems/mruby-os-memsize/mrbgem.rake
+++ b/mrbgems/mruby-os-memsize/mrbgem.rake
@@ -3,8 +3,8 @@ MRuby::Gem::Specification.new('mruby-os-memsize') do |spec|
   spec.author  = 'mruby developers'
   spec.summary = 'ObjectSpace memsize_of method'
 
-  spec.add_dependency('mruby-objectspace')
-  spec.add_test_dependency('mruby-metaprog')
-  spec.add_test_dependency('mruby-method')
-  spec.add_test_dependency('mruby-fiber')
+  spec.add_dependency('mruby-objectspace', :core => 'mruby-objectspace')
+  spec.add_test_dependency('mruby-metaprog', :core => 'mruby-metaprog')
+  spec.add_test_dependency('mruby-method', :core => 'mruby-method')
+  spec.add_test_dependency('mruby-fiber', :core => 'mruby-fiber')
 end
diff --git a/mrbgems/mruby-rational/mrbgem.rake b/mrbgems/mruby-rational/mrbgem.rake
index 534114862..2cd4816f9 100644
--- a/mrbgems/mruby-rational/mrbgem.rake
+++ b/mrbgems/mruby-rational/mrbgem.rake
@@ -3,5 +3,5 @@ MRuby::Gem::Specification.new('mruby-rational') do |spec|
   spec.author  = 'mruby developers'
   spec.summary = 'Rational class'
   spec.build.defines << "MRB_USE_RATIONAL"
-  spec.add_test_dependency('mruby-complex')
+  spec.add_test_dependency('mruby-complex', :core => 'mruby-complex')
 end