The present invention concerns a method of manufacturing flame-retardant polyethylene or polypropylene foam using a derivative of carboxylic acid of phenylphosphinic acid and the flame-retardant polymer foam made thereby. More specifically, this invention concerns a method of manufacturing highly expanded flame-retardant polymer foam prepared by mixing the above mentioned phosphorous flame retardant in a polymer resin and expanding the mixture with a volatile blowing agent or by applying a solution of an organic phosphorous compound to an already-made highly expanded foam and then drying the highly-expanded polymer foam, and the flame-retardant foam prepared thereby.