A 35-year-old male patient with Down' s syndrome received radical inguinal orchitectomy for left testicular tumor in 2006. Histological examination revealed a typical seminoma. 18 months after the operation, we found a metastasis in the right retroperitoneum lymph node. He received adjuvant chemotherapy (bleomycin, etoposide, and cisplatin : BEP). After 3 cycles of chemotherapy, the tumor disappeared. There is no obvious re-occurrence sign and general condition is good after chemotherapy. As we treated a relapse of seminoma in Down's syndrome successfully, we report this case and review 41 cases in Japanese literatures.