A 59-year-old man was referred to our hospital because of left pleural effusion detected at a regular medical checkup. A chest X-ray showed left pleural effusion. A thoracoscopic pleural biopsy specimen from the left side gave a pathological diagnosis of malignant pleural mesothelioma (MPM), epithelioid type. Then thoracoscopic exploration of the right pleura was performed because of fluorodeoxyglucose accumulation along the right dorsal pleura. Thoracoscopic exploration of the right indicated no visible tumor formation, but a pleural biopsy specimen revealed epithelioid MPM. In this case, MPM in each thorax was confined to the parietal pleura and was classified as stage I. We therefore suggest that MPM may develop simultaneously in both thoraxes.