Adenomyosis is a common disease of middle-aged women and adenomyoma is a variety of adenomyosis that formed localized tumor. Cystic degeneration of an adenomyoma is a rare clinical manifestation. A 30-year-old parous woman suffered from severe dysmenorrhea and menorrhagia for about 5 months, was operated under the impression of endometriosis of the pelvis. Following the operation, cystic degeneration of an adenomyoma was found incidentally. The authors experienced a case of adenomyosis that formed cystic tumor of uterus and presented with a pertinent literatures.