A 67-year-old-man, who had a medical history of hypertension and angina pectoris, was referred to our hospital due to dysuria and a palpable lower abdominal mass. He was a farmer until at the age of 50 and then had been working at a textile factory. The physical examination revealed a fixed, firm and round mass in the lower abdomen. The laboratory data were within the normal ranges, except for elevation of CA-125 (128 U/ml). Abdominal computed tomography (CT) and magnetic resonance imaging (MRI) showed a 6 x 5 cm round hypodense mass in the pelvic cavity. Preoperative ultrasound-guided needle biopsy resulted in a suspicious rhabdomyosarcoma. Exploratory laparotomy revealed a 10 x 8 cm size firm, round mass that was located between the superior aspect of the urinary bladder and lower anterior abdominal wall. En bloc excision of the mass with partial cystectomy was performed. The pathologic diagnosis was primary malignant peritoneal mesothelioma. The postoperative course was uneventful.