Disclosed is a compound represented by the formula (I) or a pharmacologicallyacceptable salt thereof: (I) wherein Ar represents an imidazolylgroup which may be substituted by a C1-6 alkyl group, or the like； Arrepresents a phenyl group which may be substituted by a C1-6 alkoxy group, or thelike； X represents a double bond or the like； and Het represents animidazolyl group which may be substituted by a C1-6 alkyl group or the like, orthe like. The compound or the salt is effective as a therapeutic or prophylacticagent for a disease associated with Aβ.
