A flameproofing agent for fibers which has high flameproofness and excellent physical properties (light resistance, heat resistance, texture), a process for manufacturing a flameproof fiber product and a flameproof fiber product.The flameproofing agent comprises an organic phosphorus compound (component A) represented by the following formula (1). (In the above formula, X1 and X2 are the same or different and each an aromatic substituted alkyl group represented by the following formula (2).) ALAr)n  (2) (In the above formula, AL is a branched or linear aliphatic hydrocarbon group having 1 to 5 carbon atoms, Ar is a phenyl group, naphthyl group or anthryl group all of which may have a substituent, “n” is an integer of 1 to 3, and Ar may be bonded to any carbon atom contained in AL.)