PURPOSE: To continuously obtain a synthetic resin molding filled with a fibrous reinforcing foam which is excellent in rigidity and heat insulating property, light in weight, and suitable for a building material such as sash and a structural material such as cabinet by packing a foam reinforced with long fibers into the cavity of a synthetic resin molding.

CONSTITUTION: A thermoplastic synthetic resin 2, e.g., polyvinyl chloride, etc., is molded into square pipe having a cavity 3 over the whole length from molds 13 to the longitudinal direction by means of the extruder 7 of a cross head 15, for example. A fiber bundle 10 is formed by uniformly impregnating a thermosetting expandible liquid 19, e.g., polyurethane resin, etc., into a fiber bundle 16 composed of long fibers 6, e.g., glass fibers, etc., and the fiber bundle 10 is continuously supplied into the cavity 3 of the molding 2 from a nozzle 9. The molding 2 whose cavity 3 is filled with the long fibers 6 impregnated with the expandible liquid 19 is directed into a molding pathway 11, and the expandible liquid 19 is expanded to obtain an objective product in which the long fibers 6 are uniformly dispersed in the expanded body 5 in the cavity 3 of the molding 2.