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