PURPOSE: To facilitate the formation of the titled uniform pre-expanded particles without performing any high-precision temperature control, by dispersing a specified resin material together with a volatile blowing agent in water, impregnating it with a blowing agent and releasing the water and the resin material from the system.

CONSTITUTION: A resin material is obtained by pelletizing a base material comprising a water-crosslinkable PP resin formed by chemically bonding a silyl group of the formula (wherein R is a monovalent hydrocarbon group, X is a hydrolyzable organic group and n is 0W2) to a propylene homopolymer, a copolymer of propylene with a monomer such as vinyl chloride or a graft copolymer derived by grafting styrene or the like with said homopolymer or copolymer or a mixture of this water- crosslinkable PP resin with a polyolefin resin. 100pts.wt. pellets and a volatile blowing agent (e.g., butane) are dispersed in water in the presence of 0.1W10pts.wt. dispersant (e.g., basic MgCO3) in a sealed container. This mixture is heated to 120W170°C to soften the resin particles and infiltrate the blowing agent into the particles and the particles of the resin and the water are simultaneously released under an applied pressure from the container to an outer atmosphere at a lower pressure.