A 10-year-old castrated male Siba dog was presented for signs referable to pleural effusion associated with neoplasm of the thoracic cavity. The pleural effusion fluid consisted of blood and tumor cells by thoracocentesis. Histopathological examination of the sedimentary tumor cells revealed malignant mesothelioma. Intracavitary carboplatin was administered at 300 mg/$m^2$ every 5 weeks for 3 treatment and pleural effusion was disappeared after 3 treatments. The dog had recurrence of pleural effusion at 515 days but intracavitary carboplatin chemotherapy had no effect. It would be thought that the intracavitary carboplatin treatment was quite a useful method to control a canine malignant mesothelioma with minimal toxicity.