To obtain a cellulose nano-fiber, maintaining high crystallinity under mild conditions and expectable of new uses as a material without requiring a special apparatus, in high yield in producing the cellulose nano-fiber from a cellulose raw material by an enzyme treatment.

Physical loosening is produced in a part of an aggregate of CMF (cellulose microfibril) or the CMF itself and permeability of the enzyme into the cellulose raw material is improved to simultaneously react an endoglucanase in an amorphous part produced simultaneously by the loosening. Thus, the CMF is fibrillated to afford the desired cellulose nano-fiber.