A compound represented by the formula (I):
or a pharmacologically acceptable salt thereof, wherein Ar
1
represents an imidazolyl group or the like which may be substituted with a C1-6 alkyl group, Ar
2
represents a phenyl group or the like which may be substituted with a C1-6 alkoxy group, X
1
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, is effective as a therapeutic or prophylactic agent for a disease caused by Aβ.
Disclosed is a compound represented by the formula (I-1):
or a pharmacologically acceptable salt thereof, which is effective as a therapeutic or prophylactic agent for a disease induced by Aβ, wherein X1 represents -CH=CH- and Het represents a group which may be substituted with 1 to 3 substituents selected from substituents group A1 and is represented by one of the following formulas: