A 10-year-old boy presented with a painless left scrotal mass that had been present for 2 years. Ultrasound examination revealed 2 left testes measuring 2.4 ml and 1.0 ml with the same echogenicity. Atsurgery, there were 2 similarly sized testes connecting with other spermatic vascular, but the supernumerary testis had no connection with the vas deferens. Histological findings showed immature testicular tissue without malignancy. We report the 26th case of polyorchidism in Japan and discuss this condition.