DISCLOSED IS A COMPOUND REPRESENTED BY THE FORMULA (I): OR A PHARMACOLOGICALLY ACCEPTABLE SALT THEREOF, WHEREIN AR₁ REPRESENTS AN IMIDAZOLYL GROUP WHICH MAY BE SUBSTITUTED BY A C1-6 ALKYL GROUP, OR THE LIKE, AR₂ REPRESENTS A PHENYL GROUP WHICH MAY BE SUBSTITUTED BY A C1-6 ALKOXY GROUP, OR THE LIKE, X₁ REPRESENTS A DOUBLE BOND OR THE LIKE, AND HET REPRESENTS AN IMIDAZOLYL GROUP WHICH MAY BE SUBSTITUTED BY A C1-6 ALKYL GROUP, OR THE LIKE. THE COMPOUND OR THE SALT IS EFFECTIVE AS A THERAPEUTIC OR PROPHYLACTIC AGENT FOR A DISEASE ASSOCIATED WITH Aβ.