Abstract: The present invention relates to an inhibitory agent of membrane fusion between a virus envelope and a cell membrane of a host cell for the virus, wherein the inhibitory agent comprises a serine protease inhibitor, and a therapeutic agent or a therapeutic composition, comprising the inhibitory agent. More specifically, the present invention relates to the inhibitory agent, wherein the serine protease inhibitor is one or more of a compound represented by the following general formula (I) or a salt thereof, or a solvate or hydrate thereof, and a therapeutic agent or a therapeutic composition, comprising the inhibitory agent: wherein R1 represents a substituted or unsubstituted lower alkyl group, a substituted or unsubstituted lower alkenyl group, a substituted or unsubstituted aromatic hydrocarbon group, or a substituted or unsubstituted aromatic heterocyclic group, each of which may optionally comprise a heteroatom.