PURPOSE: To obtain a regenerated polystyrene resin having a stable fire- retardant level by adding a fire retardant to a polystyrene resin before pelletizing the resin by extrusion molding to prevent the lowering of the fire retardancy of the fire-retardant polystyrene resin in a recycle process.

CONSTITUTION: In the production of a molded product composed of a fire- retardant polystyrene resin, at first, a definite amt. of a fire retardant is added to the polystyrene resin before the polystyrene resin is pelletized by extrusion molding to prevent the lowering of the fire-retardant level of the fire-retardant polystyrene resin. As the fire-retardant to be added, either one of a non-halogen type phosphoric ester compd., a halogen-containing phosphoric ester compd. and a brominated compd. being non-phosphoric ester is used. The pref. addition amt. of the fire retardant is set to 10 pts. or more with respect to 100 pts. of the fire retardant contained in the fire-retardant polystyrene resin being a starting material.