A method of preparing a precursor for a composite having wet strength of 2 to 50 kgf/mm.sup.2 and a mean surface void diameter of 0.01 to 2 .mu.m by molding a solution of a rigid polymer having a concentration of 1 to 1.3 times the optical anisotropy lower limit solution concentration C2, thereafter rendering the solution non-flowable by absorbing a coagulant under a condition causing no solvent extraction and thereafter extracting the solvent, and a method of preparing a molecular composite having a mean thickness of 2 to 200 .mu.m by impregnating this precursor for a composite with a solution of a polymer or reactive molecules.