A 57-year-old man with massive right pleural effusion was admitted to our hospital. Thoracoscopy revealed, fine granulations and small nodules scattered on the parietal pleura. Biopsy specimens suggested malignant mesothelioma. We performed thoracoscopy again one month later under general anesthesia to make a definitive diagnosis. At that time, the parietal pleura was covered with a large tumor and malignant mesothelioma was diagnosed by biopsy. We could find early pleural lesions of malignant mesothelioma in thoracoscopy. While we managed to make a definitive diagnosis, the tumor progressed rapidly during one month. If malignant pleural mesothelioma is suspected, it is necessary to make all efforts, including surgical biopsy, to diagnose during the early stage of disease.