A flame retardant resin composition includes a thermoplastic resin, flame retardant reinforced fibers, and a flame retardant. The flame retardant reinforced fibers include cellulosic fibers, a phosphorus compound that is grafted to the surface of the cellulosic fibers, and a polyamine compound that is bound to the phosphorus compound. The flame retardant is a phosphorus-based flame retardant. A method for producing the flame retardant resin composition includes the following: grafting a vinyl phosphate compound to cellulosic fibers by bringing the vinyl phosphate compound into contact with the cellulosic fibers after the cellulosic fibers have been irradiated with an electron beam, optionally followed by electron beam irradiation; binding a polyamine compound to the vinyl phosphate compound, which is grafted to the cellulosic fibers, to form flame retardant reinforced fibers; grinding the flame retardant reinforced fibers; and melt-kneading and pelletizing a thermoplastic resin, the flame retardant reinforced fibers, and a component containing a phosphorus-based flame retardant. This can provide a non-halogen flame retardant resin composition with sufficient flame resistance.