Method for producing cellulose fine fiber

Provided is a method for producing a cellulose fine fiber that has a nano size and high crystallinity and rarely undergoes the damage of a fiber shape, by impregnating cellulose with a formic acid-containing fiberizing solution and then fiberizing the cellulose, without requiring vigorous mechanical fragmentation of the cellulose. Also provided is a method for producing a surface-modified cellulose fine fiber in which the cellulose is modified. The method for producing a cellulose fine fiber according to the present invention involves impregnating cellulose with a fiberizing solution, i.e., formic acid, a formic acid-rich aqueous solution or a solution of formic acid or a formic acid-rich aqueous solution in an aprotic solvent having a number of donors of 26 or more and then fiberizing the cellulose. The method for producing a surface-modified cellulose fine fiber according to the present invention is characterized in that the fiberizing solution further contains a modification reaction agent and the method involves impregnating cellulose with the fiberizing solution and then modifying the microfibril surface of the cellulose while fiberizing the cellulose.