A 59-year-old man presented to our hospital suspected of having cholelithiasis. Computed tomography (CT) scan revealed a left retroperitoneal solid tumor cephalad to the kidney, 7 x 8 x 9 cm in size with mostly a fatty density area and focal calcification. Magnetic resonance imaging (MRI) on T1 and T2 weighted images showed a high intensity mass. Angiography revealed the hypovascular tumor. Although we had suspected it to be an adrenal myelolipoma, tumorectomy was performed because of its size. Pathological diagnosis was mature teratoma. Although retroperitoneal teratoma contains fas, cyst, soft tissue and calcification, the proportion of fat in the tumor is usually less than that of adrenal myelolipoma. In our case, the tumor contained more fat than the 'typical' retroperitoneal teratoma, which led to the misdiagnosis.