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 containing a cellulose having a cellulose I-type crystallinity of more than 33% as calculated from the following formula: Cellulose I-type Crystallinity (%)=[(I.sub.22.6-I.sub.18.5)/I.sub.22.6].times.100 wherein I.sub.22.6 is a diffraction intensity of a lattice plane (002 plane) as measured at a diffraction angle 2.theta. of 22.6.degree. in X-ray diffraction analysis; and I.sub.18.5 is a diffraction intensity of an amorphous moiety as measured at a diffraction angle 2.theta. of 18.5.degree. 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/m.sup.3, and a content of the cellulose in a residue obtained by removing water from the cellulose-containing raw material is 20% by weight or more.