There are disclosed a phosphorus-containing carboxylic acid derivative which has a group containing phosphorus atom and has in its molecule, a group represented by the following formula (1): ##STR00001## wherein R.sup.1, R.sup.2 and R.sup.3 are each hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms, and Y is oxygen atom or sulfur atom; a process for producing the derivative; and a flame retardant comprising the foregoing derivative as an effective ingredient. The present invention can provide the phosphorus-containing carboxylic acid derivative and a flame retardant which are each excellent in solubility in an organic solvent, compatibility with a variety of resins, stability and flame retardancy.