To provide a method to stabilize the physical properties of a flame retardant styrene type resin which enables not only improved physical properties of the styrene type resin but also high productivity and improved physical properties in the course of production as a result of its improved stability at melt extrusion.

With (A) a styrene type resin melt-blended is (B) a flame retordant which does not melt at 300°C or lower, at a temp. of 150-300°C after dispersing it with (C) a dispersion agent at a temp. of 0-100°C. (B) is a flame retardant selected from the group consisting of antimony oxide, a fluorinated resin and a brominated flame retardant and (C) is a higher aliph. amide and/or a liq. paraffin.