An aminocarboxylic acid derivative represented by the formula ##STR1## wherein R.sub.1 and R.sub.2, independently from each other, represent a hydrogen atom or an alkyl group which may have a substituent, and Y represents --NH-- or --S--, provided that when Y represents --NH--, at least one of R.sub.1 and R.sub.2 represents an alkyl group which may have a substituent. These compounds have excellent inhibitory activity on dipeptidase in animals, and are useful for administration in combination with carbapenem antibiotics.