An occult testicular tumor (seminoma) in a 38 year-old man with chief complaint of a left abdominal mass was reported. Laparotomy revealed a non-resectable retroperitoneal mass that was biopsied. Histologic examination of the specimen showed a seminoma. Because many of the so-called “primary extragonadal germ cell tumor” subsequently prove to have originated in the testicle, ipsilateral orchiectomy was performed. As the result, clinically quite unsuspected primary testicular neoplasma (seminoma) which would be spontaneously retrogressed was discovered by detail histologic examination of the testicular tissue. The importance of histopathologic examination of the testicle by orchiectomy was emphasized when an extragonadal germ cell tumor had been found.