An 84-year-old man who showed pleural effusion on a chest film by his family doctor consulted our hospital. A chest CT scan showed right pleural effusion, pleural nodules, and pleural thickening. For a close examination, a local anesthetic thoracoscopy (medical thoracoscopy) was performed. After removal of the pleural fluid, thoracoscopy revealed multiple nodules in parietal pleura. By its biopsy, he was diagnosed with malignant pleural mesothelioma (biphasic type). The level of adenosine deaminase (ADA) in pleural fluid was extremely high (116 IU/L). Sometimes a case of malignant pleural mesothelioma will occur with a high level of ADA. When a high level is found in pleural fluid, tuberculous pleurisy is usually suspected. But extremely high levels of ADA in pleural fluid (>100 IU/L) suggest that its causes are lymphoproliferative disorder or malignant neoplastic disease. In these situations, local anesthetic thoracoscopy (medical thoracoscopy) is a very useful diagnostic procedure to make a definite diagnosis.