Perforation of colon into the pleural space without diaphragmatic hernia is extremely rare. This report illustrates a case of pneumo-pyothorax caused by perforation of metastatic tumor of the transverse colon of a 67-year-old woman with a history of total gastrectomy and splenectomy for advanced gastric carcinoma 4 years before. The patient was admitted to our hospital presenting with fever and dyspnea, which subsided after a thoracic drainage. Cultures of drained effusion revealed Escherichia coli, Klebsiella and Bacteroides. An emergent laparotomy for treatment of mechanical ileus 2 weeks after her admission disclosed a tumor obstructing the splenic flexure of the transverse colon, and a double-barreled colostomy was made. Pathologic examination of the tumors obtained from colon, mesocolon and the parietal peritoneum revealed poorly differentiated adenocarcinoma that was the same as her primary gastric cancer.