To efficiently obtain a phenolized lignin without reducing the yield, by feeding a relatively small amount of a phenol derivative, as compared to a conventional method.

Phenol is mixed into a sulfuric acid solution and heated to dissolve the phenol in the sulfuric acid solution. Then, a kenaf core is soaked into the sulfuric acid solution and mixed to form a solid phase comprising the phenolized lignin and a liquid phase comprising other components. The solid phase is separated from the liquid phase through solid-liquid separation, subsequently neutralized and dried, to obtain the phenolized lignin.