We report here a case with malignant mesothelioma of testicular tunica vaginalis. An 81-year-old Japanese man with left hydrocele was referred for operation. When hydrocelectomy was performed, a thick wall of tunica vaginalis without malignancy was observed. Seven months after hydrocelectomy, a hard irregular mass was noticed in the left scrotum, therefore inguinal orchiectomy was performed. Pathologically, the mass showed severe atypia and mitosis. The diagnosis of malignant mesothelioma was made. He refused any adjuvant treatment and died 1 year later from multiple metastases to the paraaortic lymph nodes and lumbar supine.