A compound represented by the formula (I): or a pharmacologically acceptable salt thereof, wherein Ar1 represents an imidazolyl group or the like which may be substituted with a C1-6 alkyl group, Ar2 represents a phenyl group or the like which may be substituted with a C1-6 alkoxy group, X1 represents a double bond or the like and Het represents a triazolyl group or the like which may be substituted with a C1-6 alkyl group or the like, is effective as a therapeutic or prophylactic agent for a disease caused by Aβ.