This case study centers on a seventy-eight-year-old man with triple malignancies, namely, gastric cancer, lung cancer and malignant pleural mesothelioma, all of which developed at different times. The histological types were adenocarcinoma of the stomach, squamous cell carcinoma of the lung and sarcomatous malignant pleural mesothelioma. Gastric cancer was treated by endoscopic mucosal resection 2 years ago. The patient presented with a chief complaint of dyspnea, and right pleural effusion was found on chest radiography. The right-side effusion disappeared spontaneously, but a small mass on the left side was diagnosed as lung cancer, and so left inferior lobe resection was performed. Malignant pleural mesothelioma appeared after one year of pleural effusion and the patient died of mesothelioma one year after diagnosis. At autopsy, the gastric cancer and lung cancer had not relapsed and malignant pleural mesothelioma had metastasized to the lung, liver, adrenal gland and small intestine. He was a sailor by profession and it was obvious that he had been exposed to asbestos, because 538 asbestos bodies per 5 g of wet lung tissue were detected. His advanced age was one of the risk factors for the multiple malignancies, and the asbestos exposure was considered to have compounded these hazards to cause the triple malignancies. It is well known that lung cancer and malignant mesothelioma are induced by asbestos exposure, but multiple cancers including lung cancer and malignant mesothelioma are extremely rare.