A 45-year-old man presented with severe abdominal distention with massive ascites due to a diffusely disseminated peritoneal tumor. A core needle biopsy specimen was obtained from the peritoneal lesion. Histological diagnosis was epithelioid type mesothelioma. He did not choose to receive chemotherapy. For 2.5 years, he went without medical intervention, and his disease gradually progressed, leading to a worsening of his symptoms. The patient then chose to be treated with combination chemotherapy of cisplatin and pemetrexed, followed by pemetrexed alone. There was remarkable tumor shrinkage and his symptoms improved. These effects have been sustained for two years after the initial chemotherapy. Chemotherapy appears to have contributed to survival prolongation for this patient. This case exemplifies the fact that malignant peritoneal mesothelioma may progress slowly when fits with some good prognostic factors, and it is important to consider the prognostic factors.