A matter to be treated which contains a cellulosic polymer is heated in a solvent in the presence of an acid catalyst, to thereby selectively decompose the cellulosic polymer to a fine powder. Furthermore, a matter to be treated which contains a cellulosic polymer is reacted with an esterification agent in the presence of an acid catalyst, to thereby selectively dissolve or disperse the cellulosic polymer. In either method, the treated matter other than the cellulosic polymer does not change in state but remains solid, so that both can be easily separated by sieving or solid-liquid separation.