A novel thiazolidine derivative of general formula: ##STR1## [wherein R represents a group of general formula:G-E-D-B-A-(wherein A represents single bond, alkylene group of from 1 to 6 carbon atom(s), alkylene group of from 2 to 6 carbon atoms, a group of general formula: ##STR2## or saturated hydrocarbon ring of from 4 to 7 carbon atoms or single heterocyclic ring.Y represents alkylene group of from 1 to 4 carbon atom(s) or alkenylene group of from 2 to 4 carbon atoms.B represents single bond or alkylene group of from 1 to 6 carbon atom(s).D represents single bond, oxygen atom, carbonyl group or a group of general formula: ##STR3## R.sup.1 represents hydrogen atom, alkyl group of from 1 to 4 carbon atom(s), phenyl group or benzyl group.E represents single bond, alkylene group of from 1 to 8 carbon atom(s) or alkylene group of from 1 to 8 carbon atom(s) substituted by phenyl group or benzyl group.G represents carbocyclic or heterocyclic ring unsubstituted or substituted by from one to three of alkyl group of from 1 to 6 carbon atom(s), alkoxy group of from 1 to 6 carbon atom(s), halogen atom, trifluoromethyl group or nitro group.)With the proviso that, A and B do not represent mono-bonds at the same time.]possess inhibitory activity on prolyl endopeptidase, and therefore be useful for treating and/or preventing agent for amnesia.