A case of simultaneous double cancer on functional solitary kidney is reported. A 72-year-old man was admitted to the Department of Internal Medicine with angina, hypertension and diabetes mellitus. After treatment involving percutaneous transluminal coronary angioplasty, he was sent to the Department of Urology to check the microhematuria. Cystoscopy showed normal interior of urinary bladder. Drip infusion pyelography and computed tomography demonstrated a tumor mass on the upper pole of the right kidney and atrophic left kidney. Preoperative diagnosis was right renal cell carcinoma. However, an additional pelvic tumor was found during surgery, and a partial nephrectomy was performed. Histologic examination confirmed the presence of 2 separate and distinct malignant entities: a renal cell carcinoma and a non-invasive transitional cell carcinoma. Postoperative recovery has been uneventful and without hemodialysis for 16 months. This is the 19th case of simultaneous occurrence of renal cell carcinoma and transitional cell carcinoma in the same kidney and the 1st case in the patient with a functional solitary kidney in the Japanese literature.