A foamed polystyrene board useful as a thermal insulating material is produced by melting a polystyrene having a melt index of from 0.5 to 5 under heat in an extruder, mixing the molten mass with a blowing agent composed of methyl chloride and dichlorodifluoromethane, and extruding the resulting foamable molten gel from the extruder. As a novel essential feature, a flowability improver having a melting point higher than 70.degree. C., such as an aromatic sulfonamide, a brominated aromatic compound, a brominated alicyclic compound or a poly(.alpha.-methylstyrene), is mixed with the polystyrene within the extruder to impart superior extrusion formability. The foam has good dimensional stability both at room temperature and higher temperature, high mechanical strength and superior thermal insulation properties.