The present invention relates to a process for producing a decrystallized cellulose having a reduced cellulose I-type crystallinity from a cellulose-containing raw material in an efficient manner with an excellent productivity. In accordance with the present invention, there is provided a process for producing a decrystallized cellulose from a raw material comprising at least 20% by weight, based on the weight of the raw material excluding water contained therein, of a cellulose having a cellulose I-type crystallinity of more than 33% as calculated from the following formula: Cellulose I-type Crystallinity (%)=[(I22.6−I18.5)/I22.6]×100 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, the process including the step of treating the cellulose-containing raw material using a media-type mill to reduce the cellulose I-type crystallinity of the cellulose to 33% or less, wherein the cellulose-containing raw material has a bulk density of from 100 to 500 kg/m3.