A novel compound represented by the formula (1) below which has β-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; R1, R2 and R3 are hydrogen atom, substituted or unsubstituted alkyl group or the like, or R2 and R3 may be taken together with the adjacent nitrogen atom and carbon atom respectively to form a 3 to 6 membered ring; R4 is C1-6 alkyl group, C1-6 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.