PURPOSE: To impact excellent flame retardancy to a thermoplastic resin without detriment to the resin and without any fear of the generation of noxious gases by adding a specified polyphosphonic acid amide to the resin.

CONSTITUTION: An m-xylenediamine-type polyphenylphosphonic acid amide of the formula (wherein n is an integer of 3-150) is added to a thermoplastic resin. A preferred amide is one having a heating loss of 2wt.% of below at 300°C when it is heated at a temperature rise rate of 20°C/min in a nitrogen atmosphere in a thermobalance. A preferred thermoplastic resin is one which comprises at least one resin selected from among polyester, polyamide, polycarbonate and polyphenylene oxide.