Abstract: The invention core member is prepared by treatment of non-woven fabric with a coating composition which comprises an emulsion-form curable vinyl polymer, heat-expandable microspheres, and a thiocyanate compound, and this core member is advantageously utilized with use of liquid-form unsaturated polyester-based molding composition which contains a cobalt-containing compound as cure accelerator, wherein color development becomes available when said unsaturated polyester-based liquid composition infiltrates into voids of the core member, due to encounter of the thiocyanate compound with the cobalt-containing compound, thereby infiltration of said liquid composition is monitored by use of the color development.