A compound of formula (I) wherein Ar is an aromatic ring assembly group which may be substituted or a fused aromatic group which may be substituted; X is (i) a bond, (ii) -S-, -SO- or -SO2-, (iii) C1-6 alkylene, C2-6 alkenylene or C2-6 alkynylene, etc., (iv) -CO-O- or (v) -(CH2)p-x1-, -(CH¿2?)p-X?1-(CH¿2)q-, -(CH2)r-CO-X1-, -SO¿2-NR?8- or -(CH¿2?)r-SO2-NR?8¿- wherein X1 is O or NR8, R8 is H, a hydrocarbon group which may be substituted or an acyl, p is 0 to 5, q is 1 to 5, p+q is 1 to 5, and r is 1 to 4; Y is a divalent C¿1-6? aliphatic hydrocarbon group optionally containing O or S, which may be substituted; R?1 and R2¿ each is H or a lower alkyl which may be substituted, or R?1 and R2¿ form an N-containing heterocyclic ring which may be substituted; Ring A is a benzene ring which may be further substituted; and Ring B is a 4- to 8-membered ring which may be further substituted, or a salt thereof has the effect of inhibiting amyloid-β protein production and/or secretion and is useful as a pharmaceutical composition for preventing and/or treating the neurodegenerative disease, etc.