A novel compound represented by the formula (1) below which has ²-secretase inhibitory activity, its pharmaceutically acceptable salt or a prodrug thereof. wherein Ar is a substituted or unsubstituted 5 to 6 membered mono cyclic aromatic group； R 1 , R 2 and R 3 are hydrogen atom, substituted or unsubstituted alkyl group or the like, or R 2 and R 3 may be taken together with the adjacent nitrogen atom and carbon atom respectively to form a 3 to 6 membered ring； R 4 is C 1-6 alkyl group, C 1-6 alkyl group substituted by phenyl, phenylthio, or a hetero ring, or the like； A is represented by the formula below: wherein X and Y are oxygen atom, NH or sulfur atom, Z is hydrogen atom, hydroxy group which may be substituted, amino group, thiol group, or the like； and B is hydroxy group, substituted or unsubstituted amino group, substituted or unsubstituted aliphatic or aromatic amino group, or the like.
