A 66-year-old postmenopausal woman received routine gynecologic check-up. Transvaginal ultrasonography and abdominal and pelvic computed tomography showed about 5-cm cystic mass in uterus with solid component and the patient had thin endometrium and the serum level of CA 125 was normal. We performed a total hysterectomy and bilateral salpingo-oophorectomy and found tumor which had brownish cystic fluid and about 2 cm sized and colored in light yellowish, polypoid protruding solid mass, located within the myometrial wall. Histopathological examination of frozen section revealed malignancy. The tumor was confined within the myometrium and its histologic type was clear cell adenocarcinoma. Finally we identified that the myometrial mass was clear cell adenocarcinoma originated from adenomyosis pathologically. The malignant transformation of adenomyosis is very rare. When we find a cystic change with solid component in adenomyosis patients, clear cell adenocarcinoma should be suspected as a differential diagnosis and magnetic resonance imaging should be considered for further evaluation.