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.
