A 69-year-old man presented with left chest pain and exertional dyspnea. The chest X-ray showed massive left pleural effusion. The CT showed left pleural effusion and a tumor on the left diaphragm that was more than 8 cm and did not enhance. The MRI showed a tumor that was a low signal by T1-weighted image and a high signal by T2-weighted image. The tumor that was on the left diaphragm and along the chest wall was resected by the video-assisted thoracoscopic surgery. A postoperative histological examination revealed that this tumor was malignant. We thought it to be a leiomyosarcoma, malignant peripheral nerve sheath tumor or a sarcomatoid malignant pleural mesothelioma, but no conclusion has been reached. One year after surgery, a new lesion appeared and rapidly proliferated. A biopsy under the thoracoscope was performed. As a result, the patient was diagnosed with epitheloid malignant mesothelioma, and the chemotherapy that was begun consisted of cisplatin and pemetrexed. We again examined the first resected tumor in detail, and we concluded that it was sarcomatoid malignant mesothelioma. We considered this to be a rare case of malignant pleural mesothelioma.