PURPOSE: To obtain the titled resin particles having heat resistance and solvent resistance, by subjecting a specific amount of α-methylstyrene, a specified amount of acrylonitrile and a specific monomer to suspension polymerization with a bifunctional organic peroxide as an initiator and an easily volatile blowing agent at a specific temperature.

CONSTITUTION: 10W80wt% α-methylstyrene, 5W50wt% acrylonitrile, 0W70wt% one or more monomers of styrene, methyl methacrylate, vinyltoluene, and t- butylstyrene and a bifunctional organic peroxide (preferably producing t-butoxy radical) as an initiator are subjected to suspension polymerization at 80W130°C by adding 2W15wt% easily volatile blowing agent during polymerization or after polymerization. When high expansion is required, preferably 0.5W2.0wt% solvent or plasticizer is added to the reaction system.