A 65-year-old man had a painless mass, 30 mm in diameter, in the left scrotum. MR imaging demonstrated that the mass had low intensity in both T1-weighted and T2-weighted images. A poor enhancing effect was observed. The mass was located distinctively apart from the testis. The patient received a 12-month course of watching without medication, because these findings suggested the mass was a benign tumor. However, the tumor gradually increased in size to 50 mm in diameter during the period. Then, en bloc extirpation of the mass including the spermatic cord and testis was done via an inguinal approach. The histological diagnosis was leiomyoma of the spermatic cord. The tumor probably arose from the smooth muscle of the vessel in the spermatic cord. There has been no local recurrence during an 8-month follow-up period.