We report a rare adult case of mesoblastic nephroma. A right renal mass was detected in a 51-year-old woman by ultrasonography. The radiographic examination revealed that the mass was the hypovasucular tumor, but there was a possibility of a malignant tumor. We therefore operated on her for right radical nephrectomy. The tumor was 40 X 52 mm in size and had several cysts. Based on the histological analysis, we diagnosed it as congenital mesoblastic nephroma consisting of spindle cells.