The present invention pertains to an antiviral drug for severe fever with thrombocytopenia syndrome, which contains a compound represented by formula (I) or a salt thereof, or a solvate of the compound or salt (In the formula, R1 and R2 are the same or different from each other, and each represents a substituted or unsubstituted C1-10 alkyl group, R1 and R2 may form a substituted or unsubstituted 5- or 6-member ring in conjunction with an adjacent nitrogen atom, and X represent a halogen atom.)