We herein report a case of a renal pelvic tumor that developed in the residual left renal pelvis after right nephroureterectomy, left ureterectomy and total cystectomy in a patient with multiple urothelial tumors. The tumor was endoscopically ablated through a nephrostomy tract, and mitomycin C irrigation was carried out. We designed a novel constant-pressure irrigation system for effective and safe irrigation into the closed space of the renal pelvis. We created a hole in the urine bag tube, inserted a 5-Fr open-end ureteral catheter through the hole and kept the tip of the catheter at the end of the nephrostomy tube. The urine bag tube was placed 20 cm above the kidney level, and mitomycin C was continuously irrigated into the renal pelvis for 1 h. Six-weekly treatments were carried out, and tumor recurrence was not identified for 1 year.