Thickened uterine endometrium with abnormal uterine bleeding highly suggests endometrial hyperplasia or endometrial carcinoma. A case of 35-year-old nulliparous woman came to our department with endometrial mass manifesting as endometrial cancer. Transrectal ultrasonography and magnetic resonance imaging (MRI) showed an 8×6 cm multicystic, ill-defined mass compacted at the uterine endometrium, the anterior wall of the uterus, and 3×3 cm heterogenous mass at the left adnexa. The edometrial mass showed multiple septations with enhancement and low-signal intensity on T2-weighted images. After endometrial biopsy was done and simple hyperplasia without atypia was observed at the histopathologic finding, the patient underwent robot-assisted laparoscopy and diagnosed as adenomyoma at the frozen pathology. After adenomyomectomy, permanent pathologic analysis revealed the same result and she recovered without any complications and responded well to gonadotropin-releasing hormone (GnRH) agonist therapy.