The present invention provides compounds useful as agents for the prevention or treatment of a disease associated with abnormal plasma uric acid level and the like. The present invention relates to fused ring 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 plasma uric acid level, prodrugs thereof, salts thereof or the like. In the formula (I), X 1 and X 2 represent CH or N; ring U represents aryl or heteroaryl; m represents integral number from 0 to 2; n represents integral number from 0 to 3; R 1 represents a hydroxy group, amino or C 1-6 alkyl; R 2 represents C 1-6 alkyl, C 1-6 alkoxy C 1-6 alkyl or the like.