diff --git a/app/models/fluent_gem.rb b/app/models/fluent_gem.rb
index 6db39b8..f1124c1 100644
--- a/app/models/fluent_gem.rb
+++ b/app/models/fluent_gem.rb
@@ -22,8 +22,9 @@ module FluentGem
       # NOTE: use `fluent-gem` instead of `gem`
       Bundler.with_clean_env do
         # NOTE: this app is under the Bundler, so call `system` in with_clean_env is Bundler jail breaking
-        unless system(* [gem, *args])
-          raise GemError, "failed command #{gem} #{args.join(" ")}"
+        cmd = [gem, *args].compact
+        unless system(*cmd)
+          raise GemError, "failed command: `#{cmd.join(" ")}`"
         end
       end
       true
@@ -36,8 +37,12 @@ module FluentGem
       # On installed both td-agent and fluentd system, decide which fluent-gem command should be used depend on setup(Fluentd.instance)
       if Fluentd.instance && Fluentd.instance.fluentd?
         return "fluent-gem" # maybe `fluent-gem` command is in the $PATH
+      else
+        detect_td_agent_gem
       end
+    end
 
+    def detect_td_agent_gem
       # NOTE: td-agent has a command under the /usr/lib{,64}, td-agent2 has under /opt/td-agent
       %W(
         /usr/sbin/td-agent-gem
diff --git a/spec/models/fluent_gem_spec.rb b/spec/models/fluent_gem_spec.rb
new file mode 100644
index 0000000..10c0f8a
--- /dev/null
+++ b/spec/models/fluent_gem_spec.rb
@@ -0,0 +1,112 @@
+require 'spec_helper'
+
+describe FluentGem do
+  describe "invoker" do
+    describe "#install" do
+      let(:gem) { FluentGem.gem }
+
+      context "no argument" do
+        after { FluentGem.install }
+        it { FluentGem.should_receive(:run).with("install") }
+      end
+
+      context "with arguments" do
+        after { FluentGem.install(*args) }
+
+        context "1" do
+          let(:args) { ["plugin-foo"] }
+          it { FluentGem.should_receive(:run).with("install", *args) }
+        end
+
+        context "2" do
+          let(:args) { ["plugin-foo", "--no-document"] }
+          it { FluentGem.should_receive(:run).with("install", *args) }
+        end
+      end
+    end
+
+    describe "#uninstall" do
+      let(:gem) { FluentGem.gem }
+
+      context "no argument" do
+        after { FluentGem.uninstall }
+        it { FluentGem.should_receive(:run).with("uninstall") }
+      end
+
+      context "with arguments" do
+        after { FluentGem.uninstall(*args) }
+
+        context "1" do
+          let(:args) { ["plugin-foo"] }
+          it { FluentGem.should_receive(:run).with("uninstall", *args) }
+        end
+
+        context "2" do
+          let(:args) { ["plugin-foo", "--no-document"] }
+          it { FluentGem.should_receive(:run).with("uninstall", *args) }
+        end
+      end
+    end
+  end
+
+  describe "#list" do
+    before { FluentGem.stub(:`).and_return(gem_list) }
+    subject { FluentGem.list }
+
+    context "no list" do
+      let(:gem_list) { "" }
+      it { subject.should == [] }
+    end
+
+    context "some lines" do
+      let(:gem_list) { <<-GEM.strip_heredoc }
+        dummy (3.3.3)
+        fluent-plugin-foo (0.1.2)
+        more_dummy (0.0.1)
+      GEM
+      it { subject.should == gem_list.lines }
+    end
+
+    context "failed" do
+      let(:gem_list) { "" }
+      before { $?.stub(:exitstatus).and_return(128) }
+      it { expect{ subject }.to raise_error(FluentGem::GemError) }
+    end
+  end
+
+  describe "#run" do
+    before { FluentGem.stub(:system).and_return(ret) }
+    let(:args) { ["install", "foobar"] }
+
+    describe "success" do
+      let(:ret) { true }
+      after { FluentGem.run(*args) }
+      it { FluentGem.should_receive(:system) }
+    end
+
+    describe "failed" do
+      let(:ret) { false }
+      it { expect{ FluentGem.run(*args) }.to raise_error(FluentGem::GemError) }
+    end
+  end
+
+  describe "#gem" do
+    before { Fluentd.stub(:instance).and_return(instance) }
+    subject { FluentGem.gem }
+
+    context "any instance not setup yet" do
+      let(:instance) { nil }
+      it { should == "fluent-gem" }
+    end
+
+    context "fluentd setup" do
+      let(:instance) { Fluentd.new(id: nil, variant: "fluentd_gem", log_file: "dummy.log", pid_file: "dummy.pid", config_file: "dummy.conf") }
+      it { should == "fluent-gem" }
+    end
+
+    context "td-agent 2 setup" do
+      let(:instance) { Fluentd.new(id: nil, variant: "td_agent", log_file: "dummy.log", pid_file: "dummy.pid", config_file: "dummy.conf") }
+      it { should == FluentGem.detect_td_agent_gem }
+    end
+  end
+end