It is challenging to treat renal pelvic cancer in a monofunctioning kidney and severe renal dysfunction while preserving renal function. A 78-year-old man with renal pelvic cancer and G4 chronic kidney disease was treated with high-dose adaptive radiotherapy using helical tomotherapy with a total dose of 74 Gy in 37 fractions. His serum creatinine level before radiotherapy was 2.15 mg/dl, which temporarily increased to 5.00 mg/dl at 9 months after radiotherapy, but gradually decreased to 2.05 mg/dl 2 years later with medical treatment. Follow-up MRI showed complete tumor regression 5.5 years after radiotherapy, and dialysis was avoided. High-dose adaptive radiotherapy may be a viable option for a monofunctioning kidney and severe renal dysfunction complicated by renal pelvic cancer.