A polyglycolic acid resin is used as a molding aid to efficiently produce substantially water-insoluble thermoplastic resin moldings of various shapes, such as a porous film, ultrafine fiber, ultrathin film, and porous hollow fiber. The process comprises bringing a composite molding comprising the polyglycolic acid resin and a substantially water-insoluble thermoplastic resin into contact with anaqueous solvent to selectively remove the polyglycolic acid resin by extraction through solvolysis and obtain a molding consisting of the residual thermoplastic resin. The aqueous glycolic acid solution resulting from the extraction through solvolysis can be converted into a polyglycolic acid resin serving as a molding aid via the formation of a concentrated glycolic acid oligomer and glycolide formation.