diff --git a/src/PlantUmlClassDiagramGenerator/PlantUmlClassDiagramGenerator.csproj b/src/PlantUmlClassDiagramGenerator/PlantUmlClassDiagramGenerator.csproj
index 3dd2a9b..0658410 100644
--- a/src/PlantUmlClassDiagramGenerator/PlantUmlClassDiagramGenerator.csproj
+++ b/src/PlantUmlClassDiagramGenerator/PlantUmlClassDiagramGenerator.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 	<PropertyGroup>
 		<OutputType>Exe</OutputType>
-		<TargetFramework>net8.0</TargetFramework>
+		<TargetFrameworks>net8.0;net9.0</TargetFrameworks>
 		<PackAsTool>true</PackAsTool>
 		<ToolCommandName>puml-gen</ToolCommandName>
 		<Description>This is a generator to create a class-diagram of PlantUML from the C# source code.</Description>
diff --git a/test/PlantUmlClassDiagramGeneratorTest/PlantUmlClassDiagramGeneratorTest.csproj b/test/PlantUmlClassDiagramGeneratorTest/PlantUmlClassDiagramGeneratorTest.csproj
index 57090e2..332c326 100644
--- a/test/PlantUmlClassDiagramGeneratorTest/PlantUmlClassDiagramGeneratorTest.csproj
+++ b/test/PlantUmlClassDiagramGeneratorTest/PlantUmlClassDiagramGeneratorTest.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFramework>net8.0</TargetFramework>
+		<TargetFrameworks>net8.0;net9.0</TargetFrameworks>
 		<IsPackable>false</IsPackable>
 		<Nullable>enable</Nullable>
 	</PropertyGroup>
diff --git a/test/SourceGeneratorTest.Library/SourceGeneratorTest.Library.csproj b/test/SourceGeneratorTest.Library/SourceGeneratorTest.Library.csproj
index 42d8385..269eeff 100644
--- a/test/SourceGeneratorTest.Library/SourceGeneratorTest.Library.csproj
+++ b/test/SourceGeneratorTest.Library/SourceGeneratorTest.Library.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
diff --git a/test/SourceGeneratorTest/SourceGeneratorTest.csproj b/test/SourceGeneratorTest/SourceGeneratorTest.csproj
index 4c68f86..7bb5c4b 100644
--- a/test/SourceGeneratorTest/SourceGeneratorTest.csproj
+++ b/test/SourceGeneratorTest/SourceGeneratorTest.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFramework>net8.0</TargetFramework>
+		<TargetFrameworks>net8.0;net9.0</TargetFrameworks>
 		<ImplicitUsings>enable</ImplicitUsings>
 		<Nullable>enable</Nullable>