PURPOSE: To obtain uniform, high-ratio expansion preformed particles suitable for bead molding, by infiltrating a volatile blowing agent into an ethylene/ propylene random copolymer particles.

CONSTITUTION: A resin (mixture) particles (I) consisting of (i) 55W100wt% ethylene/propylene random copolymer and (ii) 45W0wt% low-density PE and/or ethylene/vinyl acetate copolymer and a volatile blowing agent (II), e.g., propane or butane, are dispersed in water in a closed vessel in the presence of a dispersant (III), e.g., basic magnesium carbonate, and the dispersion is heated to a temperature ≥ softening point of particles (I) to infiltrate blowing agent (II) into particles (I). The titled prefoamed particles are obtained by releaasing the particles and water simultaneously out of one end of the vessel into a low-pressure atmosphere while the pressure in the vessel is kept at a pressure ≥ vapor pressure of blowing agent (II). There prefoamed particles are aged first under normal pressure and then under an elevated pressure and then packed in a metal mold, wherein the particles are foamed by heating, fused together and formed into a molding.