The present invention provides compounds useful as agents for the prevention or treatment of a disease associated with abnormal serum uric acid level and the like. The present invention relates to (aza)indolizine derivatives represented by the following formula (I) having xanthine oxidase inhibitory activities and useful as agents for the prevention or treatment of a disease associated with abnormality of serum uric acid level, prodrugs thereof, salts thereof or the like. In the formula, 0 to 2 of X1, X2, X3 and X4 are a nitrogen atom and the others are CR1 one of T1 and T2 represents cyano and the other represents a group represented by the following formula (A), with the proviso that when T1 is cyano, at least one of X1 to X4 is a nitrogen atom R1 independently represents a hydrogen atom, a halogen atom, a hydroxy group, C1-6 alkyl, C1-6 alkoxy or the like ring U represents a benzene ring or the like m represents integral number from 0 to 2 R2 independently represents a fluorine atom, a hydroxy group or the like.
