PURPOSE: To obtain the titled fibers having high flame retardancy, by wet spinning and drawing a modacrylic polymer containing vinyl chloride and vinylidene chloride, treating the resultant gelatinous swelled fibers with an aqueous emulsion of specific colloidal antimony oxide, drying and densifying the resultant treated fibers.

CONSTITUTION: A modacrylic polymer containing vinyl chloride and vinylidene chloride is wet spun, drawn and washed with water to give fibers in a gelatinous and swelled state. The obtained fibers are then treated with an aqueous emulsion of colloidal antimony oxide prepared by reacting antimony trioxide with hydrogen peroxide, dried and then densified to afford the aimed fibers. A modacrylic fibers consisting of ≥40wt% acrylonitrile, 20W60wt% vinyl chloride or vinylidene chloride and ≤5wt% sulfonic acid group-containing monomer is preferably used as the modacrylic polymer.