diff --git a/app/controllers/articles_controller.rb b/app/controllers/articles_controller.rb
index 7ba3f2208b7..5f3f9fe963c 100644
--- a/app/controllers/articles_controller.rb
+++ b/app/controllers/articles_controller.rb
@@ -32,6 +32,8 @@ def create
     @article.user = current_user if @article.user.nil?
     set_wip
     if @article.save
+      fit_to_size_for_ogp
+
       redirect_to redirect_url(@article), notice: notice_message(@article)
     else
       render :new
@@ -41,6 +43,8 @@ def create
   def update
     set_wip
     if @article.update(article_params)
+      fit_to_size_for_ogp
+
       redirect_to redirect_url(@article), notice: notice_message(@article)
     else
       render :edit
@@ -97,4 +101,11 @@ def notice_message(article)
       article.wip? ? '記事をWIPとして保存しました' : '記事を更新しました'
     end
   end
+
+  def fit_to_size_for_ogp
+    return unless @article.thumbnail.attached?
+
+    image_resizer = ImageResizer.new(@article.thumbnail)
+    image_resizer.fit_to!(*ImageResizer::OGP_SIZE)
+  end
 end
diff --git a/app/models/article.rb b/app/models/article.rb
index b84f49aa49d..f58d0c0e360 100644
--- a/app/models/article.rb
+++ b/app/models/article.rb
@@ -4,7 +4,6 @@ class Article < ApplicationRecord
   belongs_to :user
   include ActionView::Helpers::AssetUrlHelper
 
-  THUMBNAIL_SIZE = '1200x630>'
   has_one_attached :thumbnail
 
   before_validation :set_published_at, if: :will_be_published?
@@ -21,7 +20,7 @@ class Article < ApplicationRecord
 
   def thumbnail_url
     if thumbnail.attached?
-      thumbnail.variant(resize: THUMBNAIL_SIZE).processed.url
+      thumbnail.blob.url
     else
       image_url('/images/articles/thumbnails/default.png')
     end
diff --git a/app/models/image_resizer.rb b/app/models/image_resizer.rb
new file mode 100644
index 00000000000..73dca38fa84
--- /dev/null
+++ b/app/models/image_resizer.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+class ImageResizer
+  OGP_SIZE = [1200, 630].freeze
+
+  def initialize(attached_one)
+    @attached_one = attached_one
+    @transformation = ImageResizerTransformation.new
+  end
+
+  def fit_to!(width, height)
+    return nil if just_fit_to_size?(width, height)
+
+    original_width, original_height = original_size
+
+    fitted = @attached_one.variant(**@transformation.fit_to(original_width, original_height, width, height))
+                          .processed
+                          .image.blob
+
+    # 変形前の画像関連データの削除にpurge、purge_laterは不要。
+    # データの削除はattachで担える
+    @attached_one.attach(fitted)
+  end
+
+  def just_fit_to_size?(width, height)
+    original_width, original_height = original_size
+    width == original_width && height == original_height
+  end
+
+  private
+
+  def original_size
+    blob = @attached_one.blob
+    blob.analyze unless blob.analyzed?
+    [blob.metadata[:width], blob.metadata[:height]]
+  end
+end
diff --git a/app/models/image_resizer_transformation.rb b/app/models/image_resizer_transformation.rb
new file mode 100644
index 00000000000..2a304f85f56
--- /dev/null
+++ b/app/models/image_resizer_transformation.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class ImageResizerTransformation
+  def fit_to(original_width, original_height, width, height)
+    # ハッシュを返すが処理順(リサイズ->切り抜き)にオプションを記述
+    {}.merge(resize(original_width, original_height, width, height),
+             center_crop(width, height))
+  end
+
+  # 縦横比を維持しながら、リサイズ後の縦をx、横をyとしたときに
+  # いずれかの条件を満たすようにリサイズする
+  # x = widht かつ y >= height
+  # x >= width かつ y = height
+  def resize(original_width, original_height, width, height)
+    ratio_w = width.to_f / original_width
+    ratio_h = height.to_f / original_height
+
+    if ratio_w > ratio_h
+      { resize: "#{width}x" }
+    else
+      { resize: "x#{height}" }
+    end
+  end
+
+  def center_crop(width, height)
+    { gravity: :center,
+      # 仮想キャンバスのメタデータをリセットするために!を末尾につける
+      # +repageは指定できないため、+repage相当の動作をする!を採用
+      crop: "#{width}x#{height}+0+0!" }
+  end
+end
diff --git a/app/views/articles/_form.html.slim b/app/views/articles/_form.html.slim
index f8e44c94a9e..9874fc8d4fb 100644
--- a/app/views/articles/_form.html.slim
+++ b/app/views/articles/_form.html.slim
@@ -57,7 +57,7 @@
           .a-form-help
             p
               | ここにアップロードした画像が、X（Twitter）、Facebook で投稿した際に表示される画像として使われます。
-              | 画像サイズは必ず 1200px × 630xpx でお願いします。
+              | 画像サイズが 1200px x 630px でない場合は、登録時に自動で 1200px x 630px に加工されます。
 
     - if params[:action] == 'edit'
       .form-item
diff --git a/test/fixtures/active_storage/attachments.yml b/test/fixtures/active_storage/attachments.yml
index fe637dcb6e5..d27458d4c07 100644
--- a/test/fixtures/active_storage/attachments.yml
+++ b/test/fixtures/active_storage/attachments.yml
@@ -69,3 +69,8 @@ kimuramitai_profile_image:
   name: profile_image
   record: kimuramitai (User)
   blob: kimuramitai_profile_image_blob
+
+article4_thumbnail_image:
+  name: thumbnail
+  record: article4 (Article)
+  blob: article4_thumbnail_blob
diff --git a/test/fixtures/active_storage/blobs.yml b/test/fixtures/active_storage/blobs.yml
index f1f541fc9bf..715c9a5bfe6 100644
--- a/test/fixtures/active_storage/blobs.yml
+++ b/test/fixtures/active_storage/blobs.yml
@@ -30,3 +30,6 @@ komagata_profile_image_blob: <%= ActiveStorage::Blob.fixture filename: "users/av
 machida_profile_image_blob: <%= ActiveStorage::Blob.fixture filename: "users/avatars/machida.jpg" %>
 
 kimuramitai_profile_image_blob: <%= ActiveStorage::Blob.fixture filename: "users/avatars/default.jpg" %>
+
+# article
+article4_thumbnail_blob: <%= ActiveStorage::Blob.fixture filename: "articles/ogp_images/test.jpg" %>
diff --git a/test/fixtures/articles.yml b/test/fixtures/articles.yml
index 33a0f734d27..91d4eadfe79 100644
--- a/test/fixtures/articles.yml
+++ b/test/fixtures/articles.yml
@@ -17,3 +17,9 @@ article3:
   body: 本文３
   user: komagata
   wip: true
+
+article4:
+  title: サムネイル画像付きのブログ記事
+  body: サムネイル画像付きのブログ記事
+  user: komagata
+  wip: true
diff --git a/test/models/image_resizer_test.rb b/test/models/image_resizer_test.rb
new file mode 100644
index 00000000000..6e8f104edfe
--- /dev/null
+++ b/test/models/image_resizer_test.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class ImageResizerTest < ActiveSupport::TestCase
+  test '.fit_to!' do
+    article = articles(:article4)
+
+    ImageResizer.new(article.thumbnail).fit_to!(*ImageResizer::OGP_SIZE)
+
+    article.thumbnail.blob.analyze unless article.thumbnail.blob.analyzed?
+    assert_equal({ width: 1200, height: 630 },
+                 article.thumbnail.blob.metadata.slice(:width, :height).symbolize_keys)
+  end
+
+  test '.just_fit_to_size?' do
+    article = articles(:article4)
+
+    image_resizer = ImageResizer.new(article.thumbnail)
+    width, height = ImageResizer::OGP_SIZE
+    image_resizer.fit_to!(width, height)
+
+    assert image_resizer.just_fit_to_size?(width, height)
+    assert_not image_resizer.just_fit_to_size?(width + 1, height)
+    assert_not image_resizer.just_fit_to_size?(width, height + 1)
+  end
+end
diff --git a/test/models/image_resizer_transformation_test.rb b/test/models/image_resizer_transformation_test.rb
new file mode 100644
index 00000000000..dacecbe313c
--- /dev/null
+++ b/test/models/image_resizer_transformation_test.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class ImageResizerTransformationTest < ActiveSupport::TestCase
+  setup do
+    @image_resizer_transformation = ImageResizerTransformation.new
+    @width = 1200
+    @height = 630
+  end
+
+  test '.fit_to' do
+    assert_equal({ resize: "#{@width}x", gravity: :center, crop: "#{@width}x#{@height}+0+0!" },
+                 @image_resizer_transformation.fit_to(500, 500, @width, @height))
+  end
+
+  test '.resize return 1200x when width magnification is grater than height magnification' do
+    assert_equal({ resize: "#{@width}x" }, @image_resizer_transformation.resize(500, 500, @width, @height))
+    assert_equal({ resize: "#{@width}x" }, @image_resizer_transformation.resize(500, 630, @width, @height))
+    assert_equal({ resize: "#{@width}x" }, @image_resizer_transformation.resize(1500, 1500, @width, @height))
+  end
+
+  test '.resize return x630 when height magnification is grater than width magnification' do
+    assert_equal({ resize: "x#{@height}" }, @image_resizer_transformation.resize(1000, 500, @width, @height))
+    assert_equal({ resize: "x#{@height}" }, @image_resizer_transformation.resize(1200, 500, @width, @height))
+    assert_equal({ resize: "x#{@height}" }, @image_resizer_transformation.resize(1500, 700, @width, @height))
+  end
+
+  test '.center_crop' do
+    assert_equal({ gravity: :center, crop: "#{@width}x#{@height}+0+0!" },
+                 @image_resizer_transformation.center_crop(@width, @height))
+  end
+end