A 70-year-old man with the complaint of macrohematuria and hematospermia was admitted to our hospital for further examination of a cystic formation of the right seminal vesicle, 3.6 cm in diameter, detected by magnetic resonance imaging(MRI). Cystoscopy revealed no remarkable change, but urine cytology was class III. The serum concentration of prostate specific antigen (PSA) was within the normal range of 1.83 ng/ml. Transperineal needle biopsy of the prostate and cystic tumor of the seminal vesicle revealed adenocarcinoma of the prostate and seminal vesicle, but immunostaining for PSA was negative, so we diagnosed the case as primary adenocarcinoma of the seminal vesicle. Bloody fluid of the cyst was obtained by transperineal aspiration, but no cancer cells were detected by cytological examination. Total prostatectomy was performed, and pathological findings was infiltration of prostate cancer into the seminal vesicle (pT3b) because immunostaining of the PSA was positive.