A case of adenomatoid mesothelioma of the pleura was presented. The patient was a 69-year-old woman with diabetes mellitus. She was admitted to our hospital because of a chest roentgenographic abnormality, which was pointed out during her clinical course. Chest X-ray-film on admission revealed a semicircular lesion located in the left lower lung field. The physical examination and laboratory data showed no abnormalities. Left lower lobectomy was performed. The tumor was approximately, 3.0 x 3.0 x 3.0 cm, elastic soft, encapsulated, and originated from the pulmonary pleura. The histological findings of the tumor showed benign epithelial localized pleural mesothelioma. However histologically, the tumor tissue was not like the usual epithelial type. That is, most cells which had intracellular vacuoles, were similar to signet ring cell. The tumor was diagnosed as adenomatoid mesothelioma of the pleura.