diff --git a/ChangeLog b/ChangeLog
index 5241771329..380453df6f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed Jun 29 22:04:14 2011  CHIKANAGA Tomoyuki  <nagachika00@gmail.com>
+
+	* lib/tracer.rb: Tracer.on only if required by -r command-line option.
+	  and consider --disable-gems option.
+	* test/test_tracer.rb: add tests for it.
+
 Wed Jun 29 13:55:36 2011  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* variable.c (rb_const_get_0): should not look for superclasses if
diff --git a/lib/tracer.rb b/lib/tracer.rb
index b1f08f3f68..4b2429508d 100644
--- a/lib/tracer.rb
+++ b/lib/tracer.rb
@@ -287,7 +287,12 @@ if $0 == __FILE__
   ARGV.shift
   Tracer.on
   require $0
-elsif caller.count {|bt| /\/rubygems\/custom_require.rb:/ !~ bt} <= 1
-  Tracer.on
+else
+  # call Tracer.on only if required by -r command-line option
+  count = caller.count {|bt| /\/rubygems\/custom_require.rb:/ !~ bt}
+  if (defined?(Gem) and count == 0) or
+     (!defined?(Gem) and count <= 1)
+    Tracer.on
+  end
 end
 # :startdoc:
diff --git a/test/test_tracer.rb b/test/test_tracer.rb
index 8f542e6e84..a1e0a4571c 100644
--- a/test/test_tracer.rb
+++ b/test/test_tracer.rb
@@ -1,14 +1,15 @@
 require 'test/unit'
+require 'tmpdir'
 require_relative 'ruby/envutil'
 
 class TestTracer < Test::Unit::TestCase
   include EnvUtil
 
-  def test_work_with_e
-    assert_in_out_err(%w[--disable-gems -rtracer -e 1]) do |(*lines),|
+  def test_tracer_with_option_r
+    assert_in_out_err(%w[-rtracer -e 1]) do |(*lines),|
       case lines.size
       when 2
-        assert_match %r[#0:<internal:lib/rubygems/custom_require>:\d+:Kernel:<: -], lines[0]
+        assert_match(%r{rubygems/custom_require\.rb:\d+:Kernel:<:}, lines[0])
       when 1
         # do nothing
       else
@@ -17,4 +18,46 @@ class TestTracer < Test::Unit::TestCase
       assert_equal "#0:-e:1::-: 1", lines.last
     end
   end
+
+  def test_tracer_with_option_r_without_gems
+    assert_in_out_err(%w[--disable-gems -rtracer -e 1]) do |(*lines),|
+      case lines.size
+      when 1
+        # do nothing
+      else
+        flunk "unexpected output from `ruby --disable-gems -rtracer -e 1`"
+      end
+      assert_equal "#0:-e:1::-: 1", lines.last
+    end
+  end
+
+  def test_tracer_with_require
+    Dir.mktmpdir("test_ruby_tracer") do |dir|
+      script = File.join(dir, "require_tracer.rb")
+      open(script, "w") do |f|
+        f.print <<-EOF
+require 'tracer'
+1
+        EOF
+      end
+      assert_in_out_err([script]) do |(*lines),|
+        assert_empty(lines)
+      end
+    end
+  end
+
+  def test_tracer_with_require_without_gems
+    Dir.mktmpdir("test_ruby_tracer") do |dir|
+      script = File.join(dir, "require_tracer.rb")
+      open(script, "w") do |f|
+        f.print <<-EOF
+require 'tracer'
+1
+        EOF
+      end
+      assert_in_out_err(["--disable-gems", script]) do |(*lines),|
+        assert_empty(lines)
+      end
+    end
+  end
 end