The development of therapeutic medicines against the newly pathogenic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is desired considering the current COVID-19 pandemic. However, developing a highly efficient antiviral drug against a new virus in which virus-type mutations occur frequently or whose characteristics are unknown properties is difficult. Therefore, using existing antiviral drugs with a broad spectrum and certain effectiveness for various viruses is currently the first choice for effective drug therapy. To identify a promising candidate drug to treat viral infection, especially for influenza, hepatitis C, and now COVID-19, several RNA nucleobase analogs such as remdesivir, 2,3 ribavirin, brincidofovir, and favipiravir: 6-fluoro-3-hydroxypyrazine-2-carboxamide (favipiravir, T-705), sold under the brand name avigan, have been developed.