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 A&bgr；.本發明提供一種由式(I)所代表之化合物：；(I)；或其藥理學上可接受之鹽類，其中Ar1代表可經由C1－6烷基或類似基團所取代之咪唑基，Ar2代表可經由C1－6烷氧基或類似基團所取代之苯基，X1代表雙鍵或類似結構，以及Het代表可經由C1－6烷基或類似基團所取代之咪唑基，其可有效地作為由Aβ所引起之疾病的治療或預防藥劑。
