[wherein n represents an integer of from 1 to 4, R
1
represents substituted or unsubstituted lower alkyl, —C(═Y)R
9
or the like, R
2
represents a hydrogen atom, substituted or unsubstituted lower alkyl or the like, R
3
represents substituted or unsubstituted lower alkyl, substituted or unsubstituted aryl, a substituted or unsubstituted heterocyclic group or the like, and R
4
represents halogen or the like.]A phosphodiesterase 10A inhibitor which comprises a quinoline derivative represented by the above formula (I) or a pharmaceutically acceptable salt thereof as an active ingredient is provided.