Provided is a compound which is useful as a prophylactic or therapeutic agent for diseases or the like caused by abnormal serum urate level. An (aza)indolizine derivative represented by formula (I), said (aza)indolizine derivative having a xanthine oxidase inhibitory activity and being useful as a prophylactic or therapeutic agent for diseases caused by abnormal serum urate level, a prodrug thereof or a salt, etc. of the same. In formula (I), 0 to 2 of X1 to X4 represent a nitrogen atom and the remainder represent CR1； one of T1 and T2 represents cyano while the other represents a group represented by formula (A) [provided that when T1 is cyano, at least one of X1 to X4 represents a nitrogen atom]； R1s independently represent a hydrogen atom, a halogen atom, a hydroxyl group, C1-6 alkyl, C1-6 alkoxy, etc.； the ring U represents a benzene ring, etc.； m is an integer of 0 to 2； and R2s independently represent a fluorine atom, a hydroxyl group, etc.
