diff --git a/LvglProjectFileUpdater/LvglWindowsLibraryProjectUpdater.cs b/LvglProjectFileUpdater/LvglWindowsLibraryProjectUpdater.cs
index 34277ad..366fb92 100644
--- a/LvglProjectFileUpdater/LvglWindowsLibraryProjectUpdater.cs
+++ b/LvglProjectFileUpdater/LvglWindowsLibraryProjectUpdater.cs
@@ -216,7 +216,13 @@ public static void Run()
 
             foreach (var CurrentName in NewSourceNames)
             {
-                ProjectRoot.AddItem("ClCompile", CurrentName.Item1);
+                {
+                    ProjectItemElement Item =
+                        ProjectRoot.AddItem("ClCompile", CurrentName.Item1);
+                    Item.AddMetadata(
+                        "AdditionalOptions",
+                        "/utf-8 %(AdditionalOptions)");
+                }
 
                 {
                     ProjectItemElement Item =
diff --git a/LvglProjectFileUpdater/Program.cs b/LvglProjectFileUpdater/Program.cs
index 7ea3541..d6ae6aa 100644
--- a/LvglProjectFileUpdater/Program.cs
+++ b/LvglProjectFileUpdater/Program.cs
@@ -217,7 +217,13 @@ static void UpdateLvglWindowsSimulator()
 
             foreach (var CurrentName in NewSourceNames)
             {
-                ProjectRoot.AddItem("ClCompile", CurrentName.Item1);
+                {
+                    ProjectItemElement Item =
+                        ProjectRoot.AddItem("ClCompile", CurrentName.Item1);
+                    Item.AddMetadata(
+                        "AdditionalOptions",
+                        "/utf-8 %(AdditionalOptions)");
+                }
 
                 {
                     ProjectItemElement Item =
@@ -354,7 +360,13 @@ static void UpdateLvglWindowsDesktopApplication()
 
             foreach (var CurrentName in NewSourceNames)
             {
-                ProjectRoot.AddItem("ClCompile", CurrentName.Item1);
+                {
+                    ProjectItemElement Item =
+                        ProjectRoot.AddItem("ClCompile", CurrentName.Item1);
+                    Item.AddMetadata(
+                        "AdditionalOptions",
+                        "/utf-8 %(AdditionalOptions)");
+                }
 
                 {
                     ProjectItemElement Item =