The present invention relates to a thiazole derivative represented by formula (I)
wherein Ar1 represents a substituted or unsubstituted phenyl or thienyl group, Y1 and Y2 are different and each represents a nitrogen atom or a sulfur atom, R1 represents a hydrogen atom, a C1 to C5 alkyl group, a phenyl group, or an amino group unsubstituted or mono- or di-substituted with a C1 to C5 alkyl group, and R2 represents a substituted or unsubstituted nitrogen heterocycle group, or a pharmaceutically acceptable salt thereof. The compound of the present invention are a dopamine D4 receptor antagonist which has an antipsychotic effect without causing any extrapyramidal disorder.