A flame retardant additive comprises (A) a gassing agent which is a compound containing both phosphorus and nitrogen, typically ammonium polyphosphate, which is surface coated with (B) a silicone-base water repellent treating agent comprising a co-hydrolytic condensate obtained through co-hydrolytic condensation of (i) an organosilicon compound and (ii) an amino group-containing alkoxysilane or a partial hydrolyzate thereof in the presence of an organic or inorganic acid. The additive is improved in moisture absorption resistance and dispersion and suited for use in resins or elastomers.