An excellent β secretase inhibitor is provided, which comprises a compound of the general formula: wherein Ar is an aromatic group; X is a divalent group selected from —O—, —S—, —CO—, —SO—, —SO2—, —NR8—, CONR8—, —SO2NR8—, and —COO— (wherein R8 is hydrogen, etc.), a divalent C1-6 aliphatic hydrocarbon group which may contain one or two of these divalent groups, or a bond; Y is a divalent group selected from —O—, —S—, —CO—, —SO—, —SO2—, —NR8—, —CONR8—, —SO2NR8—, and —COO—, or a divalent C1-6 aliphatic hydrocarbon group which may contain one or two of these divalent groups; R1 and R2 are hydrogen, a hydrocarbon group, etc., respectively; and A is a ring which may be further substituted, or a salt thereof.