The invention relates to compounds of the following formula (I) or their salts: <；CHEM>； in which R<；1>； represents OR<；4>； or others, in which R<；4>； is an alkyl group having 1-8 carbon atoms which may have a substituent or the like； R<；2>； is halogen, nitro, cyano, carboxyl, or the like； R<；3>； is hydrogen, halogen, hydroxyl, amino, carboxyl, or the like； X is NR<；11>；, oxygen, or sulfur, in which R<；11>； is hydrogen, or an alkyl group having 1-8 carbon atom which may have a substituent； and each of Y and Z is CR<；12>； or nitrogen, in which R<；12>； has the same meaning as R<；3>； above, and a xanthine oxidase inhibitor containing the compound as an active ingredient.
