diff --git a/app/models/fluentd/agent/common.rb b/app/models/fluentd/agent/common.rb
index c101261..a0f752c 100644
--- a/app/models/fluentd/agent/common.rb
+++ b/app/models/fluentd/agent/common.rb
@@ -76,6 +76,25 @@ class Fluentd
         end
       end
 
+      def config_merge(content)
+        if content.start_with?("<label ")
+          label = content.slice(/<label\s+(.+?)>/, 1)
+          key = "label:#{label}"
+          parsed_config = parse_config(config)
+          if parsed_config.key?(key)
+            offset = parsed_config[key][0][:pos] + parsed_config[key][0][:size]
+            label, sections = parse_label_section(content, offset)
+            parsed_config[key][0][:sections]["filter"].concat(sections["filter"])
+            parsed_config[key][0][:sections]["match"].concat(sections["match"])
+            config_write(dump_parsed_config(parsed_config))
+          else
+            config_append(content)
+          end
+        else
+          config_append(content)
+        end
+      end
+
       def configuration
         if File.exists? config_file
           ::Fluentd::Agent::Configuration.new(config_file)
@@ -160,7 +179,7 @@ class Fluentd
           when "label"
             label_content = header + scanner.scan_until(%r{^</label>})
             label, sections = parse_label_section(label_content, started)
-            contents["label:#{label}"] << { label: label, pos: started, sections: sections }
+            contents["label:#{label}"] << { label: label, pos: started, sections: sections, size: label_content.size }
           else
             raise TypeError, "Unknown section: #{started}: #{section_type}"
           end
diff --git a/test/models/fluentd/agent_common_test.rb b/test/models/fluentd/agent_common_test.rb
index 7280ddd..3432865 100644
--- a/test/models/fluentd/agent_common_test.rb
+++ b/test/models/fluentd/agent_common_test.rb
@@ -80,7 +80,7 @@ class Fluentd
             { pos: 0, content: source_section_content }
           ],
           "label:@INPUT" => [
-            { label: "@INPUT", pos: 60, sections: sections }
+            { label: "@INPUT", pos: 60, sections: sections, size: 116 }
           ]
         }
         assert_equal(expected, actual)
@@ -157,10 +157,10 @@ class Fluentd
             { pos: 61, content: source_section_content2 },
           ],
           "label:@INPUT" => [
-            { label: "@INPUT", pos: 124, sections: input_sections }
+            { label: "@INPUT", pos: 124, sections: input_sections, size: 136 }
           ],
           "label:@MAIN" => [
-            { label: "@MAIN", pos: 260, sections: main_sections }
+            { label: "@MAIN", pos: 260, sections: main_sections, size: 211 }
           ]
         }
         assert_equal(expected, actual)
@@ -186,5 +186,68 @@ class Fluentd
         assert_equal(fixture_content("config/multi-label.conf"), config)
       end
     end
+
+    sub_test_case "#config_merge" do
+      test "simple" do
+        stub(@agent).config { fixture_content("config/simple.conf") }
+        stub(@agent).backup_config
+        content = <<-CONFIG
+<match dummy3>
+  @type stdout
+</match>
+        CONFIG
+        mock(@agent).config_append(content)
+        @agent.config_merge(content)
+      end
+
+      test "simple with label" do
+        stub(@agent).config { fixture_content("config/simple.conf") }
+        stub(@agent).backup_config
+        content = <<-CONFIG
+<label @INPUT>
+  <match dummy3>
+    @type stdout
+  </match>
+</label>
+        CONFIG
+        mock(@agent).config_append(content)
+        @agent.config_merge(content)
+      end
+
+      test "append to label" do
+        stub(@agent).config { fixture_content("config/label.conf") }
+        stub(@agent).config_file { "tmp/fluent.conf" }
+        stub(@agent).backup_config
+        content = <<-CONFIG
+<label @INPUT>
+  <match dummy3>
+    @type stdout
+  </match>
+</label>
+        CONFIG
+        mock(@agent).config_write(<<-CONFIG)
+<source>
+  @type dummy
+  tag dummy
+  @label @INPUT
+</source>
+
+<label @INPUT>
+  <filter dummy>
+    @type stdout
+  </filter>
+
+  <match dummy>
+    @type stdout
+  </match>
+
+  <match dummy3>
+    @type stdout
+  </match>
+</label>
+        CONFIG
+        @agent.config_merge(content)
+      end
+    end
   end
 end