Amino acid derivatives of formula (I), which specifically inhibit prolylendopeptidase activity and are useful for treating or preventing dementia and amnesia, wherein Q represents W-(CH2)m-(A)n-CO-B-, A represents oxygen or imino, B represents (II) or -(CH2)l-, X and Y represent each sulfur or methylene, W represents (III, IV) or (V), wherein Hal represents halogen, R represents hydrogen, formyl, C1 to C5 alkoxycarbonyl, cyano, or -CR1 = N - R2, R1 represents hydrogen, R2 represents hydroxy, C¿1? to C5 alkylcarboxy, phenylcarboxy, C1 to C5 alkoxy, or phenyl(C1 to C5) alkoxy, and n, m and l represent integers of, respectively, 0 or 1, 0 to 3, and 1 to 3, provided that W represents (VI) when R represents hydrogen, formyl or C1 to C5 alkoxycarbonyl.