The invention relates to compounds of the following formula (I) or their salts: in which R 1 is a group of phenyl, naphthyl, pyridyl or the like which may have a substituent such as an alkyl group having 1-8 carbon atoms, an alkoxy group having 1-8 carbon atoms, an alkoxycarbonyl group having 2-8 carbon atoms, carboxyl, halogen, hydroxyl, nitro, or cyano； R 2 is cyano, nitro, or the like； R 3 is hydroxyl or the like； X is oxygen, sulfur, or the like； and Y is oxygen or sulfur, and a xanthine oxidase inhibitor containing the compound.
