A 39-year-old woman was admitted to our hospital because a hard mass about 3cm in diameter with a tender was found in a scar of cesarean section. Ultrasonography (US) showed a 22.9×14.9×24.5 mm hypoechoic and heterogeneous mass with an irregular margin. Abdominal contrast-enhanced CT scan showed an enhanced mass in the abdominal wall. The US-guided needle biopsy of the mass was performed for the histological diagnosis and the mass turned out to be endometriosis that was positive staining for vimentin, estrogen receptor and CD10 by immunohistochemistory. Measures available for the treatment of endometriosis are hormone therapy and surgical resection. In our case, surgical resection was performed completely, because preoperative diagnosis of the disease was made from a needle biopsy. It is important to avoid extended operation and to employ appropriate therapies combined with hormone and surgical therapy to maintain the patient's quality of life, because endometriosis is a benign disease per se.