The invention relates to a highly productive process for producing decrystallized cellulose which includes treating a cellulose-containing raw material by means of a mill, wherein the cellulose-containing raw material has a cellulose content of a residue obtained by removing water from the cellulose-containing raw material of 20 mass% or more, has a cellulose I-type crystallinity of cellulose more than 33% as calculated from the following formula (1): Cellulose I-type Crystallinity%=I22.6-I18.5/I22.6x100 wherein I22.6 is a diffraction intensity of a lattice plane (002 plane) as measured at a diffraction angle 2θ of 22.6° in X-ray diffraction analysis and I18.5 is a diffraction intensity of an amorphous moiety as measured at a diffraction angle 2θ of 18.5° in X-ray diffraction analysis, and has a water content of 1.8 mass% or less, to thereby reduce the cellulose 1-type crystallinity to 33% or less.
