The present invention relates to a compound represented by Formula (I): (wherein Ar1 represents an imidazolyl group which may be substituted with 1 to 3 substituents; Ar2 represents a pyridinyl group, a pyrimidinyl group, or a phenyl group which may be substituted with 1 to 3 substituents; X1 represents (1) —C≡C— or (2) a double bond etc. which may be substituted; R1 and R2 represent, for example, a C1-6 alkyl group or C3-8 cycloalkyl group which may be substituted) or a pharmacologically acceptable salt thereof and to the use thereof as pharmaceutical agents. The object of the present invention is to find a therapeutic or preventive agent for diseases caused by Aβ. According to the present invention, a therapeutic or preventive agents for diseases caused by Aβ can be provided.