A pre-expanded beads of olefin-modified polystyrene-based resin comprising a pie-expanded beads of a polystyrene-based resin modified with a polyolefin-based resin, wherein a styrene-based monomer forming a polystyrene-based resin in the beads is used in the range of 100 to 1,000 parts by weight relative to 100 parts by weight of a polyolefin-based resin, a bulk density of each bead is 0.012 to 0.20 g/cm.sup.3, and an absorbance ratio at 698 cm.sup.-1 and 2850 cm.sup.-1 (D.sub.698/D.sub.2850) obtained from an infrared absorption spectrum of each bead surface measured by ATR method infrared spectroscopy is in the range of 0.1 to 2.5.