A compound of the formula: wherein Ring A and Ring B are (1) Ring A is an optionally substituted unsaturated monocyclic heterocyclic ring, and Ring B is an optionally substituted unsaturated monocyclic heterocyclic ring, an optionally substituted unsaturated fused heterobicyclic ring, or an optionally substituted benzene ring, (2) Ring A is an optionally substituted benzene ring, and Ring B is an optionally substituted unsaturated monocyclic heterocyclic ring, an optionally substituted unsaturated fused heterobicyclic ring, or an optionally substituted benzene ring, or (3) Ring A is an optionally substituted unsaturated fused heterobicyclic ring, wherein —NR— group and —CH2— group are both on the same ring of the unsaturated fused heterobicyclic ring, and Ring B is an optionally substituted monocyclic unsaturated heterocyclic ring, an optionally substituted unsaturated fused heterobicyclic ring, or an optionally substituted benzene ring; and R is a hydrogen atom, a lower alkyl group, a lower alkanoyl group or a lower alkoxycarbonyl group, or a pharmaceutically acceptable salt thereof, or a prodrug thereof.