The object of the present invention is to provide apharmaceutical composition for preventing or treatingviral infectious diseases. The compounds of the presentinvention have extremely potent anti-HCV activity and HCVgrowth inhibitory effects, and since they also onlydemonstrate mild cytotoxicity in vivo, a pharmaceuticalcomposition containing the compound of the presentinvention is extremely useful as an anti-HCVpreventive/therapeutic agent.