A 34-year-old man visited our hospital complaining of a small painless left scrotal mass. His serum alpha-fetoprotein and human chorionic gonadotropin-beta levels were normal. Ultrasonography revealed a solitary 14 mm mass. Magnetic resonance imaging revealed a mass with high intensity on T2-weighted imaging. Computed tomography revealed a heterogeneous tumor in the left scrotum. Left high orchiectomy was performed. The histopathological diagnosis was a teratoma without germ cell neoplasia in situ (GCNIS). Fluorescence in situ hybridization analysis showed no appearance of i(12p). The patient was clinically diagnosed as having a prepubertal-type testicular teratoma. Adult teratomas contain GCNIS and are aggressively treated as malignant germ cell tumors. However, a prepubertal-type teratoma is benign and does not relapse. It is essential to validate the appearance of i(12p) to differentiate prepubertal and postpubertal-type teratoma.