A 76-year-old man came to the department of gastrointestinal medicine with lower left abdominal discomfort and constipation. A crossed fused ectopic kidney with a renal tumor in the left upper pole of the kidney was detected by computed tomography. We performed left partial nephrectomy safely in spite of the complicated shape and complexity of the blood vessels. The pathological diagnosis was clear cell renal cell carcinoma, pT3a, with a negative surgical margin. After surgery, renal function was well preserved.