@bernhard Thanks again, but the error was caused when compiling. Here I make a sample repo. I think this line should not makes any error whether ProGuard works or doesn't, but cause "Unresolved reference: staticMethod". https://github.com/appiritskwatanabe/ProGuardSampleApp/blob/master/app/src/main/kotlin/com/sample/proguardsampleapp/MainActivity.kt#L21 How can I preserve the library's Top-Level Function? I wrote the name of the class which include the function in the lib's proguard-rules.pro, but no luck.