An energy absorber suitable for use as a bumper core or the like is formed of an expansion-molded article of prefoamed particles. The prefoamed particles use, as the base resin thereof, a resin obtained by impregnating high-density polyethylene with an aromatic vinyl monomer and polymerizing the monomer. The absorber has a density ( rho ) of 0.015-0.15 g/cm3 and satisfies the following relation: E20/ rho >/=45 kgxcm/g wherein E20 (kgxcm/cm3) means the quantity of energy absorbed upon 70% compression at 20 DEG C. and rho has the same meaning as defined above.
