ltd which is an aromatic polyphosphate having the structure indicated in the chemical formula 2 as a phosphoric ester based flame retardant was used as a raw material the fiber strength and the flame retardancy of the fiber obtained are shown in table 1 the fiber strength is 5 77 cn dtex and the mean value of 5 samples is 4 6 times and the firing time is a high value of 16 seconds which means excellent flame retardancy a polypropylene multi filament yarn was obtained under the same conditions as in example 1 except that only a polypropylene base resin having a melt flow rate value of 30 g 10 min was used the nor type hals based stabilizer and the phosphoric ester based flame retardant were not contained and the stretching magnification was changed to 4 46 the fiber strength of the fiber obtained is 6 03 cn dtex and high but the flame retardancy is extremely low in a mean value of 1 2 times a polypropylene multi filament yarn was obtained under the same conditions as in example 1 except that the contents of the nor type hals based stabilizer and the phosphoric ester based flame retardant and the stretching magnification were changed.