The present invention provides a pharmaceutical composition that has excellent safety and targets a different step in the virus propagation cycle than conventional pharmaceutical compositions, applicable as an antiviral agent. A pharmaceutical composition containing an epigallocatechin gallate derivative represented by the following chemical formula (1), an isomer thereof, or a salt thereof is prepared. This can be used as a membrane fusion inhibitor that inhibits viral membrane fusion. In the following formula, R1 to R6 are each a hydrogen atom, halogen, sodium, potassium, or a straight-chain or branched, saturated or unsaturated acyl group and may be identical to or different from one another. The acyl group may be substituted further with one or more substituents. At least one of R1 to R6 is the acyl group. R7 to R16 are each a hydrogen atom, halogen, sodium, or potassium and may be identical to or different from one another.