diff --git a/KoeBook/ViewModels/MainViewModel.cs b/KoeBook/ViewModels/MainViewModel.cs
index 14f9ac4..bda5ed5 100644
--- a/KoeBook/ViewModels/MainViewModel.cs
+++ b/KoeBook/ViewModels/MainViewModel.cs
@@ -4,6 +4,7 @@
 using KoeBook.Core.Contracts.Services;
 using KoeBook.Core.Models;
 using KoeBook.Services;
+using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
 using Windows.Storage.Pickers;
 using WinRT.Interop;
@@ -27,7 +28,14 @@ public sealed partial class MainViewModel : ObservableRecipient
     [NotifyCanExecuteChangedFor(nameof(StartProcessCommand))]
     private string? _ebookUrl;
 
-    private bool CanExecuteStartProcess => EbookFilePath is not null || !string.IsNullOrEmpty(EbookUrl);
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(ErrorTextVisibility))]
+    [NotifyPropertyChangedFor(nameof(CanExecuteStartProcess))]
+    private bool _ebookIsValid = true;
+
+    public Visibility ErrorTextVisibility => EbookIsValid ? Visibility.Collapsed : Visibility.Visible;
+
+    private bool CanExecuteStartProcess => EbookFilePath is not null || !string.IsNullOrEmpty(EbookUrl) && EbookIsValid;
 
     [ObservableProperty]
     private bool _skipEdit = true;
@@ -135,4 +143,28 @@ static async void CoreAsync(ILocalSettingsService settingsService, bool skipEdit
             await settingsService.SaveSettingAsync(SkipEditSettingsKey, skipEdit);
         }
     }
+
+    partial void OnEbookUrlChanged(string? value)
+    {
+        EbookIsValid = IsValid(value);
+
+        static bool IsValid(string? value)
+        {
+            if (string.IsNullOrEmpty(value))
+                return true;
+            ReadOnlySpan<string> allowedOrigins = [
+                "https://www.aozora.gr.jp"
+            ];
+
+            try
+            {
+                var uri = new Uri(value);
+                return allowedOrigins.Contains(uri.GetLeftPart(UriPartial.Authority));
+            }
+            catch (UriFormatException)
+            {
+                return false;
+            }
+        }
+    }
 }
diff --git a/KoeBook/Views/MainPage.xaml b/KoeBook/Views/MainPage.xaml
index d282124..15af6f8 100644
--- a/KoeBook/Views/MainPage.xaml
+++ b/KoeBook/Views/MainPage.xaml
@@ -54,11 +54,17 @@
                     </StackPanel>-->
 
                     <StackPanel>
-                        <TextBlock Text="青空文庫を読み上げる"/>
+                        <TextBlock Text="Webページを読み上げる (青空文庫のみ対応)"/>
                         <TextBox
                             Text="{x:Bind ViewModel.EbookUrl, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                             BeforeTextChanging="{x:Bind ViewModel.BeforeTextChanging}"
-                            Margin="{StaticResource XSmallTopMargin}" />
+                            Margin="{StaticResource XSmallTopMargin}"
+                            PlaceholderText="https://www.aozora.gr.jp/cards/000035/files/1567_14913.html"/>
+                        <TextBlock
+                            Visibility="{x:Bind ViewModel.ErrorTextVisibility, Mode=OneWay}"
+                            Text="青空文庫のURLを入力してください"
+                            Style="{ThemeResource CaptionTextBlockStyle}"
+                            Foreground="{ThemeResource SystemErrorTextColor}"/>
                     </StackPanel>
 
                     <Button