An antiviral drug contains as its effective component an isothiosemicarbazone represented by the following general formula (I) or an acidic salt thereof: ##STR1## wherein R.sub.1 represents a halogen atom, a lower (C.sub.1 -C.sub.6) alkyl group, a hydroxy group, a phenyl group which may be or may not be substituted with an alkoxy group containing 1 to 12 carbon atoms, or an unsubstituted or substituted phenylethynyl group represented by the following general formula (II): ##STR2## (wherein R.sub.5 and R.sub.6 may be the same or different, and they each represent a hydrogen atom, a halogen atom, a lower (C.sub.1 -C.sub.6) alkyl group, a hydroxy group or an alkoxy group containing 1 to 12 carbon atoms); R.sub.2 represents a hydrogen atom or a lower (C.sub.1 -C.sub.6) alkyl group; R.sub.3 represents a lower (C.sub.1 -C.sub.6) alkyl group or a benzyl group; R.sub.4 represents a hydrogen atom or a lower (C.sub.1 -C.sub.6) alkyl group. Many of these effective component compounds not only have antiviral activity (especially against influenza virus), but also prevent the breeding of and exterminate plant pathogenic molds.