Incidentally, the bulk expansion ratio is a value determined by placing a plurality of the foamed beads in a graduated cylinder, dividing the weight of the foamed beads by a volume read by the graduated cylinder at this time to find a bulk density (g/cm3) of the foamed beads, and dividing the density (g/cm3) of the base resin of the foamed beads by the thus-found bulk density of the foamed beads.