An 81-year-old man was admitted to our hospital with abdominal distension due to refractory ascites of unknown origin. He subsequently died of aspiration pneumonia. Autopsy revealed a diagnosis of biphasic malignant peritoneal mesothelioma (MPM) containing both epithelioid and sarcomatous components. The diagnosis of MPM is often difficult because serum tumor markers, imaging studies, and the cytology of ascites may not provide enough information. Accordingly, peritoneal biopsy is necessary in order to diagnose MPM based on the histological and immunohistochemical findings.