[Object] The object of the present invention is to provide a molded foam product in which a flame retardant having no accumulating property in a living body and satisfactory handling property during the production process is dissolved in a plasticizer so that the flame retardant can be uniformly impregnated into resin particles, and which shows excellent heat-fusibility between the particles during molding without resulting in uneven cell density upon expanding, and has excellent flame retardancy and heat resistance.[Means of Realizing the Object] The expandable polystyrene resin particles are prepared by supplying a flame-retardant solution to the aqueous suspension before or during impregnating polystyrene resin particles dispersed in an aqueous suspension with a blowing agent, in which the flame-retardant solution is prepared by dissolving in a plasticizer 33 to 1,000 parts by weight of powdery flame retardant relative to 100 parts by weight of the plasticizer and 20 to 200 parts by weight of flame retardant aid having a 1-hour half-life temperature of 100 to 250° C. relative to 100 parts by weight of the plasticizer and impregnating the polystyrene resin particles with the flame retardant and the flame retardant aid, wherein the powdery flame retardant comprises tetrabromocyclooctane that is preliminarily dispersed by silica fine powders and the silica fine powders are contained in 0.3 to 1.5 parts by weight relative to 98.5 to 99.7 parts by weight of the tetrabromocyclooctane.