PURPOSE:To manufacture a foamed polystyrene molded part with the wall part, the thickness of which is smaller than the specified value, by a method wherein a cavity, which is formed by a cavity mold and a core mold, is evacuated and, after that or while the cavity is being sucked, specified primary foamed particles are loaded together with compressed air in the cavity. CONSTITUTION:First, while an evacuating device is put into actuation, a charging port 9 is opened so as to carry expandable polystyrene beads, the particle diameter of which is 0.1-0.35mm, with compressed air into a cavity in order to load the beads in the cavity. Secondly, molds are preheated by introducing heating medium such as steam, hot water or the like through heating and cooling medium inlets 7 and 12 into chambers 5 and 10. While the heating medium is kept on introducing, steam is introduced through steam blowing ports 4 into the cavity for main heating. After the finish of main heating, cooling medium such as water or the like is introduced in the chambers 5 and 10 in order to cool the molds. After the completion of cooling, the molds are opened and compressed air is blown through releasing air blowing ports 14 so as to release a molded part with the wall thickness of 0.6-1.2mm from a core mold 2.