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