diff --git a/web/backend/app/models/converter/text_to_moon.rb b/web/backend/app/models/converter/text_to_moon.rb
index df0427b6..a10fa2cc 100644
--- a/web/backend/app/models/converter/text_to_moon.rb
+++ b/web/backend/app/models/converter/text_to_moon.rb
@@ -8,10 +8,10 @@ class TextToMoon
     CONVERT_SIZE = 4
 
     attribute :text, :string
-    attribute :size, :integer, default: 20
+    attribute :size, :integer, default: 10
 
     validates :text, presence: true
-    validates :text, length: { minimum: 1, maximum: 20 }
+    validates :text, length: { minimum: 1, maximum: 10 }
     validates :text, format: { with: /\A[ぁ-んァ-ンa-zA-Z0-9ー０-９Ａ-Ｚａ-ｚ 　]+\z/, message: '変換できるのは全角半角英数字、ひらがなカタカナのみです。' }
     validates :size, numericality: { only_integer: true, greater_than_or_equal_to: 20, less_than_or_equal_to: 100 }
 
diff --git a/web/backend/spec/models/converter/text_to_moon_spec.rb b/web/backend/spec/models/converter/text_to_moon_spec.rb
index d807d0f1..3fb6d015 100644
--- a/web/backend/spec/models/converter/text_to_moon_spec.rb
+++ b/web/backend/spec/models/converter/text_to_moon_spec.rb
@@ -11,30 +11,30 @@
 
     context 'textに半角全角英数字、ひらがなカタカナの文字が含まれているとき' do
       it 'ひらがなのみなのでエラーにならないことを確認' do
-        text_to_moon = Converter::TextToMoon.new(text: 'あいうえお', size: 20)
+        text_to_moon = Converter::TextToMoon.new(text: 'あいうえお', size: 10)
         expect(text_to_moon.valid?).to be true
       end
 
       it 'カタカナのみなのでエラーにならないことを確認' do
-        text_to_moon = Converter::TextToMoon.new(text: 'アイウエオ', size: 20)
+        text_to_moon = Converter::TextToMoon.new(text: 'アイウエオ', size: 10)
         expect(text_to_moon.valid?).to be true
       end
 
       it '半角英数字のみなのでエラーにならないことを確認' do
-        text_to_moon = Converter::TextToMoon.new(text: 'abcde12345', size: 20)
+        text_to_moon = Converter::TextToMoon.new(text: 'abcde12345', size: 10)
         expect(text_to_moon.valid?).to be true
       end
 
       it '全角英数字のみなのでエラーにならないことを確認' do
-        text_to_moon = Converter::TextToMoon.new(text: 'ＡＢＣＤＥ１２３４５', size: 20)
+        text_to_moon = Converter::TextToMoon.new(text: 'ＡＢＣＤＥ１２３４５', size: 10)
         expect(text_to_moon.valid?).to be true
       end
 
       it 'textとsizeに受け取ったパラメータが設定されていることを確認' do
-        text_to_moon = Converter::TextToMoon.new(text: 'ＡＢＣＤＥ１２３４５', size: 20)
+        text_to_moon = Converter::TextToMoon.new(text: 'ＡＢＣＤＥ１２３４５', size: 10)
         response = text_to_moon.call
         expect(response[:text]).to eq 'ＡＢＣＤＥ１２３４５'
-        expect(response[:size]).to eq 20
+        expect(response[:size]).to eq 10
       end
 
     end
diff --git a/web/frontend/src/pages/TopPage.vue b/web/frontend/src/pages/TopPage.vue
index a2aad515..6afa83d5 100644
--- a/web/frontend/src/pages/TopPage.vue
+++ b/web/frontend/src/pages/TopPage.vue
@@ -10,7 +10,7 @@ import { computed } from "vue";
 import { ComputedRef } from "vue";
 import { reactive } from "vue";
 
-const size = ref<number>(20);
+const size = ref<number>(10);
 const text = ref<string>("");
 const isValid = ref<boolean>(false);
 const response = ref<string[]>([]);
@@ -37,10 +37,10 @@ const inputNumberOnly = (inputValue: string): boolean | string => {
   const regex = /^[0-9]+$/;
   return regex.test(inputValue) || "数字を入力してください";
 };
-const inputNumber20to100 = (inputValue: string): boolean | string => {
-  // 20 ~ 100の数字のみの入力を許可する
-  const regex = /^[2-9][0-9]$|^100$/;
-  return regex.test(inputValue) || "20 ~ 100の数字を入力してください";
+const inputNumber10to100 = (inputValue: string): boolean | string => {
+  // 10 ~ 100の数字のみの入力を許可する
+  const regex = /^[1-9][0-9]$|^100$/;
+  return regex.test(inputValue) || "10 ~ 100の数字を入力してください";
 };
 
 const inputText1to20 = (inputValue: string): boolean | string => {
@@ -83,9 +83,9 @@ onErrorCaptured((err: Error) => {
               <v-form @submit.prevent v-model="isValid">
                 <v-text-field
                   v-model="size"
-                  :rules="[inputNumberOnly, inputNumber20to100]"
+                  :rules="[inputNumberOnly, inputNumber10to100]"
                   label="月のサイズ"
-                  placeholder="20 ~ 100"
+                  placeholder="10 ~ 100"
                 ></v-text-field>
 
                 <v-text-field