To obtain a flame retardant resin composition obtain by using a rubber reinforced resin containing a graft copolymer prepared by using a specific emulsifier, excellent in process fluidity, mechanical strength as well as flame retardancy and useful in the OA field, etc.

In the production process of a graft polymer by grafting one or more kinds of vinyl compounds to a rubber-like polymer, as at least one of emulsifier, is used an emulsifier bearing 21 radically polymerizable double bond in its molecule [preferably a compound of formulas I and/or II (M is an alkali metal, an alkaline earth metal; (n) is a positive integer of 1-100) to give a rubber-reinforced resin containing a radical copolymer. (A) 100 pts.wt. of this rubber-reinforced resin is combined with (B) 0.1-30 pts.wt. of a flame retardant to prepare the objective resin composition. As the component (B), an organophosphorus compound and/or a halogenated organic substance, particularly a halogenated epoxy oligomer is preferably used.