A melamine polymetaphosphate having a solubility of from 0.01 to 0.10 g/100 ml in water at 25.degree. C., a pH of from 2.5 to 4.5 in the form of a 10 wt % aqueous slurry at 25.degree. C. and a melamine content of from 1.0 to 1.1 mol per mol of phosphorus atom.A process for producing such a melamine polymetaphosphate, which comprises the following steps (a) and (b):(a) a step of mixing melamine, urea and an aqueous orthophosphoric acid solution containing at least 40 wt % of orthophosphoric acid, in such a ratio that melamine is from 1.0 to 1.5 mol per mol of orthophosphoric acid and urea is from 0.1 to 1.5 mol per mol of orthophosphoric acid, at a temperature of from 0 to 140.degree. C., to form a reaction mixture, and stirring the reaction mixture at a temperature of from 0 to 140.degree. C., while removing water, to obtain a powdery product of a double salt of orthophosphoric acid with melamine and urea; and(b) a step of calcining the powdery product obtained in step (a) at a temperature of from 240 to 340.degree. C. for from 0.1 to 30 hours, while preventing agglomeration, to obtain a melamine polymetaphosphate.