PROBLEM TO BE SOLVED: To provide a polystyrene foam insulator in which a chemical effective for control of insects or the like but safe for men and beasts is dispersed with concentration uniform and effective to the inside, and the chemical effects can be permanently preserved. SOLUTION: A sodium borate aqueous solution is thermoregulated to an enough temperature to cause at least partial prefoaming of polystyrene beads at the following process. At least partial prefoaming of the polystyrene beads is caused by fluidly introducing the beads into a vessel while the thermoregulated sodium borate aqueous solution is sprayed. Water content is removed from the prefoamed polystyrene beads by evaporation, and sodium borate is solidly stuck to the minutely uneven surface of the plystyrene beads. The complete foaming is established by introducing the sodium borate-carried and prefoamed polystyrene beads into the mold and by heating it. A foamed polystyrene block is molded with making the surface tacky and integratedly fusing it by small pressure.
