A polyolefin resin drawn sheet 4 is laminated on at least one face of a sheet-form core material 6 having a density of 30 to 300 kgf/m.sup.3. At this time, a sheet or film 5 made of a bonding synthetic resin or rubber having a flow starting temperature lower than the thermal deformation temperature of the core material 6 and the melting point of the drawn sheet 4 is interposed between the core material 6 and the drawn sheet 4. The resultant stack product is heated to not less than the flow starting temperature of the synthetic resin or the rubber and not more than the thermal deformation temperature of the core material 6 and the melting point of the drawn sheet 4. At the same time of or after the heating, the stack product is pressed to apply a compression strain of 0.01 to 10% to the core material 6.