Nanofiber that keep crystallinity are obtained from a cellulose material under light load. The nanofiber are obtained by treating a material containing cellulose with 4% by mass or more and 9% by mass or less of an aqueous alkali metal hydroxide solution to produce alkali cellulose, reacting the alkali cellulose with carbon disulfide to give cellulose xanthate, and defibrating the cellulose xanthate. Then, the xanthate is treated with acid or heat to be regenerated into cellulose nanofiber.