The present invention provides a tri-substituted pyrimidine compound having an excellent PDE10 inhibitory activity. The present invention relates to a tri-substituted pyrimidine compound represented by the following formula [I] or a pharmaceutically acceptable salt thereof, a method for preparing the same, and use of said compound for PDE10 inhibitor, and a pharmaceutical composition comprising said compounds as an active ingredient: wherein: either one of X and X is N, and the other of X and X is CH； A is *-CH=CH-, *-C(Alk)=CH-, *-CH-CH- or *-O-CH- (* is a bond with R)； Alk is a lower alkyl group； Ring B is an optionally substituted nitrogen-containing aliphatic heterocyclic group； R is an optionally substituted quinoxalinyl or an optionally substituted quinolyl； Y is mono- or di- substituted amino group, or a pharmaceutically acceptable salt thereof.
