It is intended to provide a novel compound that has an excellent γ-secretase inhibitory effect and specifically inhibits Aβ production.The present invention provides a compound of the following formula (1) or a pharmaceutically acceptable salt thereof: wherein R1 represents a linear or branched alkyl group having 1 to 4 carbon atoms or a phenyl group; R2 represents a linear or branched alkyl group having 1 to 4 carbon atoms which may be substituted by one or more phenyl or halogenophenyl groups; R3 represents a linear or branched alkyl group having 1 to 4 carbon atoms which may be substituted by one or more hydroxyl groups; R4 represents a linear or branched alkyl group having 1 to 4 carbon atoms; and the symbol “*” represents a chiral center.