Idiopathic retroperitoneal fibrosis (IRF) is characterized by the progressive proliferation of connective tissue, but it rarely results in the formation of a mass. Herein, we report a rare case of tumor-forming IRF. A 76-year-old woman was referred to our hospital after a tumor in the right retroperitoneum was found by ultrasonography and computed tomography. Magnetic resonance imaging showed a 5 x 8 x 5-cm irregularly shaped tumor, lying adjacent to the right kidney, with a high-intensity T1-weighted image and a high-intensity T2-weighted image. Hormonal levels were within normal limits. Surgery was performed because of the possibility of an adrenal cancer. The tumor was firm, measured 7 x 8 x 4 cm, and weighed 115 g. The pathological diagnosis was retroperitoneal fibrosis. It is very difficult to distinguish tumor-forming IRF from malignancy. Several examinations, including needle aspiration cytology and biopsy, are necessary for the diagnosis and treatment of this disease.