Wilson's disease shows considerably various symptoms that sometimes timely diagnosis is delayed when patient shows vague clinical presentation. We present a female patient whose initial symptom was hypersomnia and severe fatigue. She was initially diagnosed as depression. Because initial hepatic function test was unremarkable, it was not easy to come to think about relationship between hepatic function and hypersomnia. Her final diagnosis was Wilson's disease. This case suggested that hypersomnia otherwise unexplained could be the presenting symptom of Wilson's disease.