We report a case of carcinoma in situ of urothelial carcinoma (UC) developing in an ileal neobladder. Some cases of secondary UC in the ileal segment of a urinary diversion have been reported. However, none were cases of carcinoma in situ. After cystectomy for bladder cancer, a 73-year-old man developed UC at the left ureteral-neobladder and urethral-neobladder anastomoses. Left nephroureterectomy, resection of the neobladder, and total urethrectomy were performed. The pathologic examination revealed UC (papillary, grade 3) at these anastomoses and carcinoma in situ in the ileal neobladder.