Polystyrene-based composite resin particles includes 100 to 500 parts by weight of a polystyrene-based resin with respect to 100 parts by weight of an ethylene-vinyl acetate copolymer resin. The polystyrene-based composite resin particles are impregnated with a volatile blowing agent and are pre-expanded to obtain pre-expanded particles; the obtained pre-expanded particles are immersed in tetrahydrofuran for 24 hours to obtain an extract A; and the pre-expanded particles are divided into two halves through the center to prepare halved particles, and the halved particles are immersed in tetrahydrofuran for 24 hours to obtain an extract B; and the obtained extract A and extract B are subjected to a GPC (gel permeation chromatography) measurement and obtain the following results: the polystyrene-based resin having a weight-average molecular weight (Mw) of 100,000 to 500,000 gives a peak obtained from chromatograms for the extract A and for the extract B.