diff --git a/tool/rbinstall.rb b/tool/rbinstall.rb
index f910fca..2f21884 100755
--- a/tool/rbinstall.rb
+++ b/tool/rbinstall.rb
@@ -942,7 +942,7 @@ def load_gemspec(file, base = nil)
 end
 
 def install_default_gem(dir, srcdir, bindir)
-  gem_dir = Gem.default_dir
+  gem_dir = ENV['GEM_DESTDIR']
   install_dir = with_destdir(gem_dir)
   prepare "default gems from #{dir}", gem_dir
   RbInstall.no_write do
@@ -991,7 +991,7 @@ def install_default_gem(dir, srcdir, bindir)
 end
 
 install?(:ext, :comm, :gem, :'bundled-gems') do
-  gem_dir = Gem.default_dir
+  gem_dir = ENV['GEM_DESTDIR']
   install_dir = with_destdir(gem_dir)
   prepare "bundled gems", gem_dir
   RbInstall.no_write do