This invention is intended to produce a novel functional material through solubilization and molecular weight reduction of a water-insoluble polymeric compound, such as a water-insoluble protein or water-insoluble polysaccharide, in a simple and efficient manner. This invention provides a method for producing a degradation product of a water-insoluble polymeric compound comprising the steps of: bringing a water-insoluble polymeric compound into contact with a solid acid catalyst, heating the resulting mixture, and recovering a supernatant; adding an aqueous medium to the solid acid catalyst after the supernatant is recovered, agitating and heating the resulting mixture, and recovering a supernatant; washing the solid acid catalyst with an aqueous medium and recovering a wash solution; mixing the recovered supernatant with the wash solution, so as to obtain a fraction that has not adsorbed to the solid acid catalyst; and eluting an adsorbed fraction from the solid acid catalyst and recovering an eluate, so as to obtain a fraction that has adsorbed to the solid acid catalyst.