PURPOSE: To enhance productivity by making a foamability imparting process unnecessary, by preparing a foamed molded body using foamed particles obtained by extrusion method.

CONSTITUTION: A resin composition containing an olefinic resin and an expanding agent is extruded from a perforated die to be foamed and cut to obtain foamed particles wherein foaming magnification is 35 times or more, a closed cell ratio is 65% or more, a foamed cell diameter is 30W200μm and a particle size is 2W20 mm. The foamed particles are allowed to successively fill the cavity of a mold pressurized to 0.2W6.0 kg/cm2G by compressed gas while compressed using compressed gas having pressure 0.5 kg/cm2 or more higher than that in the mold and divided into plural times. The pressure in the mold is continuously held to the aforementioned pressure in the mold during filling and returned to atmospheric pressure after filling is finished to introduce steam into the mold and foamed particles are heated to be fused to each other and, at this time, the compression ratio of the foamed particles is controlled to 3W65%. By this method, a molded body of olefinic resin foamed particles having a high closed cell structure is obtained in the mold.