Adenosarcoma of the uterine body is a rare mixed tumor in which a benign epithelial component is mixed with a malignant stromal element. It has been considered that this tumor originates from the endometrium and its most common finding of imaging is a polypoid tumor occupying the uterine cavity. The authors herein present a case of 37-year-old female with a complaint of abnormal vaginal bleeding. At the first visit, transvaginal ultrasound and magnetic resonance imaging (MRI) showed a round mass with a diameter of one cm in the uterine wall. No malignant pathological finding was detected. The patient visited the authors again one year later, because of continuous bleeding. At that time, they found a polypoid tumor in the uterine cavity, which turned out to be adenosarcoma with sarcomatous overgrowth. The round mass in the uterus detected at first time seems to have been incipience of adenosarcoma. Prodromal sign of adenosarcoma has not been reported previously.