Favipiravir is a new antiviral drug against influenza. It is metabolized into favipiravir ribosyl triphosphate (favipiravir RTP) by an intracellular enzyme, and favipiravir RTP selectively inhibits RNA polymerase (RNA-dependent RNA polymerase) of the influenza virus, preventing replication of the influenza virus. It is a drug with a mechanism of action different from that of the existing influenza antiviral drugs and effective against all types and sub-types of human influenza A, B, and C viruses in vitro, showing a wide range of anti-viral activity against various influenza virus strains including avian and swine viruses.