A case of sweat gland carcinoma arising in a mature cystic teratoma in a 63-year-old woman is reported. The patient was admitted with the complaint of a lower abdominal pain. Under the diagnosis of a mature cystic teratoma of the right ovary, the tumor was removed. Histologically, adenocarcinoma was found in the wall of the mature cystic teratoma. A part of the tumor showed a differentiation of the secretory portion and duct of the sweat gland. Histochemically, sialic acid containing mucoprotein was defected. Electron microscopically, the tumor cells were found to have secretory granules and intracytoplasmic canaliculi. Therefore, the tumor was diagnosed as a sweat gland carcinoma arising from a mature cystic teratoma.