diff --git a/spec/qiita/markdown/filters/code_block_spec.rb b/spec/qiita/markdown/filters/code_block_spec.rb
new file mode 100644
index 0000000..3356a6f
--- /dev/null
+++ b/spec/qiita/markdown/filters/code_block_spec.rb
@@ -0,0 +1,126 @@
+# frozen_string_literal: true
+
+describe Qiita::Markdown::Filters::CodeBlock do
+  subject(:filter) { described_class.new(input_html) }
+
+  let(:context) { nil }
+
+  context "without code" do
+    let(:input_html) do
+      <<~HTML
+        <pre>
+        </pre>
+      HTML
+    end
+
+    it "does not change" do
+      expect(filter.call.to_s).to eq(input_html)
+    end
+  end
+
+  context "with code" do
+    let(:input_html) do
+      <<~HTML
+        <pre><code>
+        </code></pre>
+      HTML
+    end
+
+    it "does not change" do
+      expect(filter.call.to_s).to eq(input_html)
+    end
+
+    context "with data-metadata" do
+      let(:input_html) do
+        <<~HTML
+          <pre><code data-metadata>
+          </code></pre>
+        HTML
+      end
+
+      it "does not change" do
+        expect(filter.call.to_s).to eq(input_html)
+      end
+
+      context "with data-metadata value" do
+        let(:input_html) do
+          <<~HTML
+            <pre><code data-metadata="ruby">
+            </code></pre>
+          HTML
+        end
+
+        let(:output_html) do
+          <<~HTML
+            <pre lang="ruby"><code data-metadata="ruby">
+            </code></pre>
+          HTML
+        end
+
+        it "adds lang on pre" do
+          expect(filter.call.to_s).to eq(output_html)
+        end
+
+        context "with value include filename" do
+          let(:input_html) do
+            <<~HTML
+              <pre><code data-metadata="ruby:abc.rb">
+              </code></pre>
+            HTML
+          end
+
+          let(:output_html) do
+            <<~HTML
+              <pre filename="abc.rb" lang="ruby"><code data-metadata="ruby:abc.rb">
+              </code></pre>
+            HTML
+          end
+
+          it "adds lang and filename on pre" do
+            expect(filter.call.to_s).to eq(output_html)
+          end
+        end
+      end
+
+      context "with data-metadata value like filename" do
+        let(:input_html) do
+          <<~HTML
+            <pre><code data-metadata="abc.rb">
+            </code></pre>
+          HTML
+        end
+
+        let(:output_html) do
+          <<~HTML
+            <pre filename="abc.rb" lang="ruby"><code data-metadata="abc.rb">
+            </code></pre>
+          HTML
+        end
+
+        it "adds lang and filename on pre" do
+          expect(filter.call.to_s).to eq(output_html)
+        end
+      end
+
+      context "with data-metadata value like filename without extension" do
+        let(:input_html) do
+          <<~HTML
+            <pre><code data-metadata="Dockerfile">
+            </code></pre>
+          HTML
+        end
+
+        let(:output_html) do
+          <<~HTML
+            <pre lang="Dockerfile"><code data-metadata="Dockerfile">
+            </code></pre>
+          HTML
+        end
+
+        it "adds lang and filename on pre" do
+          expect(filter.call.to_s).to eq(output_html)
+        end
+      end
+    end
+  end
+end