We report a case of granulomatous prostatitis after intravesical BCG immunotherapy for superficial bladder cancer. A 58-year-old man presented with gross hematuria. Cystoscopic examination revealed multiple tumors at the posterior wall of the bladder. The patient underwent transurethral resection of the tumor. Intravesical BCG immunotherapy was postoperatively followed and it eradicated the disease. Digital examination revealed that the prostate became stony-hard and larger 10 weeks after the initial BCG immunotherapy. A needle aspiration cytology and biopsy of the prostate revealed the granulomatous prostatitis due to BCG immunotherapy.