The present invention is to provide a composition for preventing plant diseases which comprises at least Component I and Component II, is capable of obtaining a synergic effect that cannot be expected with each single component, is capable of markedly increasing a control effect in a much smaller amount of chemicals against various plant pathogens, and causes no phytotoxicity suffering. A composition for preventing plant diseases comprises Components I and II as active ingredients. The Component I is (RS)--N--[2-(1,3-dimethylbutyl)thiophen-3-yl]-1-methyl-3-tri fluoromethyl-1H-pyrazole-4-carboxamide. As the Component II, there can be exemplified, for example, tetraconazole, flutriafol, imibenconazole, triadimefon, simeconazole, oxpoconazole fumarate, prothioconazole, bupirimate, spiroxamine, metiram, dodine, anilazine, chlozolinate, oxycarboxin, ethaboxam, iprovalicarb, pyrazophos, fluoroimide, diflumetorim, fenhexamid, famoxadone, fenamidone, cyazofamid, zoxamide, cyflufenamid, boscalid, benthiavalicarb-isopropyl, picoxystrobin, pyraclostrobin, fluoxastrobin or dimoxystrobin.