A thiazolylimidazole derivative represented by the formula or a pharmaceutically acceptable salt thereof, and an ALK5 inhibitor, an therapeutic agent for alopecia or a hair growth agent having the above as an active ingredient, wherein: X1 and X2 are different from each other and represent a sulfur atom or a carbon atom; R1 represents a phenyl group; a substituted phenyl group; a phenyl group condensed with a hetero aromatic ring; a pyridyl group; or a pyridyl group condensed with a hetero aromatic ring; R2 represents a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkyl group having 1 to 6 carbon atoms substituted with 1 to 5 halogen atoms, an alkoxy group having 1 to 6 carbon atoms, an alkanoyl group having 1 to 5 carbon atoms, or a hydroxyalkyl group having 1 to 6 carbon atoms, A represents a group which is represented by the formula. The present invention provides an inhibitory substance against ALK5 which is a TGF-β type I receptor and provides a hair growth stimulant or a hair growth agent based on its novel activities.