The present invention relates to a compound represented by formula [I]: wherein: R1 is hydrogen, halogen, lower alkyl or cyano; Ring A is an optionally substituted heterocyclic group; Ring B is an optionally substituted 3 to 6-membered monocyclic group; and Y is optionally substituted amino, optionally substituted cyclic amino, optionally substituted aliphatic 3 to 6-membered monocyclyloxy, optionally substituted lower alkyl or optionally substituted lower alkyl-O—, or a pharmaceutically acceptable salt thereof, and to their use as PDE10 inhibitor.