We report a case of diabetic mastopathy in a man with type 2 diabetes. The patient was a 62-year-old man who had been diagnosed with type 2 diabetes at the age of 46 years. He had been treated with oral hypoglycemic agents. He noticed a mass in his left breast in February 2007, when HbA₁c was 7.6% with the treatment using oral hypoglycemic agents, including acarbose, glimepiride, buformine, and pioglitazone. Mammography of the breast showed increased density, and ultrasonography showed a regular-shaped hypoechoic mass. Core needle biopsy was performed, and diabetic mastopathy was confirmed pathologically. Diabetic mastopathy usually occurs in women with type 1 diabetes. This case, a man with type 2 diabetes, is very rare.