We report a case of malignant peritoneal mesothelioma (MPM) in a 63-year-old man. He had body weight loss and abdominal distension for one month, and was admitted to our hospital. Abdominal sonography showed a large mass occupying the right lower abdomen and an existence of a lot of ascites. Computed tomography and magnetic resonance image showed a lot of ascites and omentum cake. Cytology of the ascites was Class V but its histological classification was unknown. Then we performed biopsy of the tumor into the omentum with abdominal sonography. The histological diagnosis was MPM because the tumor cells showed positive for calretinin. He received a combination chemotherapy of weekly intra-peritoneal administration of cisplatin (70 mg/ day) and paclitaxel (100 mg/day). The ascites was decreased and per os (PO) was possible, but omentum cake was not changed. MPM was poor in prognosis and the control of ascites was difficult. We suggest that the chemotherapy of intra-peritoneal administration was a better procedure than others to control ascites with malignant tumors.