diff --git a/CHANGES.md b/CHANGES.md
index 3e3cede..4d1ede7 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -10,6 +10,12 @@ To be released.
 
  -  The `uri` type has completly gone; use `url` instead.
     [[#126], [#281] by Jonghun Park]
+ -  Support [external_tag][etag] via '@external-tag' annotation.
+    [[#266], [#316] by JaeHyung Jang]
+
+[etag]: https://serde.rs/enum-representations.html
+[#266]: https://github.com/nirum-lang/nirum/issues/266
+[#316]: https://github.com/nirum-lang/nirum/pull/316
 
 ### Docs target
 
diff --git a/docs/serialization.md b/docs/serialization.md
index 5a66b5c..05c174e 100644
--- a/docs/serialization.md
+++ b/docs/serialization.md
@@ -250,6 +250,27 @@ It's represented in JSON to:
 In a similar way to a record type, undefined fields in a payload are ignored
 by deserializer.
 
+If '@external-tag' annotation is given on tag below example:
+
+     union name =
+         @external-tag
+         east-asian-name ( text family-name, text given-name )
+         | ...
+         ;
+
+It's representated in JSON to:
+
+     {
+        {
+            "east-asian-name": {
+                    "_type": "name",
+                    "_tag": "east-asian-name",
+                    "family_name": "Hong",
+                    "given_name": "Minhee"
+                }
+            }
+        }
+    }
 
 Option type
 -----------
diff --git a/examples/shapes.nrm b/examples/shapes.nrm
index 9d4197f..1ee1e81 100644
--- a/examples/shapes.nrm
+++ b/examples/shapes.nrm
@@ -37,7 +37,8 @@ union shape
           point upper-left,
           point lower-right
       )
-    | circle (point origin, offset radius)
+    | @external-tag
+      circle (point origin, offset radius)
     ;
 
 // This is comment.  Note that there are important differences between
diff --git a/src/Nirum/Targets/Python.hs b/src/Nirum/Targets/Python.hs
index 6d1e5c4..585c01e 100644
--- a/src/Nirum/Targets/Python.hs
+++ b/src/Nirum/Targets/Python.hs
@@ -436,7 +436,8 @@ class $funcName(object):
     |]
 
 compileUnionTag :: Source -> Name -> Tag -> CodeGen Markup
-compileUnionTag source parentname d@(Tag typename' fields' _) = do
+compileUnionTag source parentname d@(Tag typename' fields' annotations') = do
+    let hasExternalTag = M.member "external-tag" $ AI.annotations annotations'
     abc <- collectionsAbc
     fieldCodes <- toFieldCodes source
         (\ f -> [qq|value.get('{toBehindSnakeCaseText (fieldName f)}')|])
@@ -521,7 +522,7 @@ class #{className}(#{parentClass}):
 %{ endcase }
 
     def __nirum_serialize__(self):
-        return {
+        t = {
             '_type': '#{behindParentTypename}',
             '_tag': '#{behindTagName}',
 %{ forall fc@FieldCode { fcField = Field { fieldType = fType } } <- fieldCodes }
@@ -529,6 +530,10 @@ class #{className}(#{parentClass}):
 #{compileSerializer' source fType $ T.append "self." $ fcAttributeName fc},
 %{ endforall }
         }
+%{ if hasExternalTag }
+        t = {'#{behindTagName}': t}
+%{ endif }
+        return t
 
     @classmethod
 %{ case pyVer }
@@ -544,6 +549,13 @@ class #{className}(#{parentClass}):
     ) -> typing.Optional['#{className}']:
 %{ endcase }
         handle_error = #{defaultErrorHandler}(on_error)
+%{ if hasExternalTag }
+        try:
+            value = '#{toBehindSnakeCaseText typename'}'
+        except KeyError:
+            handle_error('.', 'Expected #{toBehindSnakeCaseText typename'} to exist.')
+            handle_error.raise_error()
+%{ endif }
         if isinstance(value, #{abc}.Mapping):
             try:
                 tag = value['_tag']
diff --git a/test/nirum_fixture/fixture/foo.nrm b/test/nirum_fixture/fixture/foo.nrm
index 43603fa..1f283b3 100644
--- a/test/nirum_fixture/fixture/foo.nrm
+++ b/test/nirum_fixture/fixture/foo.nrm
@@ -144,3 +144,8 @@ record name-shadowing-field-record (
 union optional-union = foo ( int32? bar )
                      | baz ( int32 qux )
                      ;
+
+union external-tag-union = tag ( int32 tag )
+                         | @external-tag
+                           etag ( int32 etag)
+                         ;
\ No newline at end of file
diff --git a/test/serialization/unions/external-tag.json b/test/serialization/unions/external-tag.json
new file mode 100644
index 0000000..1950707
--- /dev/null
+++ b/test/serialization/unions/external-tag.json
@@ -0,0 +1,18 @@
+{
+    "description": "Serialize into external tag when gives 'external-tag' annotation",
+    "type": "fixture.foo.etag",
+    "input": {
+        "etag": {
+            "_type": "external_tag_union",
+            "_tag": "etag",
+            "etag": 1
+        }
+    },
+    "normal": {
+        "etag": {
+            "_type": "external_tag_union",
+            "_tag": "etag",
+            "etag": 1
+        }
+    }
+}