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 % = I 22.6 - I 18.5 / I 22.6 x 100 wherein I 22.6 is a diffraction intensity of a lattice plane (002 plane) as measured at a diffraction angle 2¸ of 22.6° in X-ray diffraction analysis and I 18.5 is a diffraction intensity of an amorphous moiety as measured at a diffraction angle 2¸ 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.본 발명은 셀룰로오스 함유 원료에서 물을 뺀 잔여 성분 중의 셀룰로오스 함유량이 20질량% 이상이고, 하기 계산식(1)로 표시되는 셀룰로오스의 셀룰로오스 I형 결정화도가 33%를 넘으면서, 수분함량이 1.8질량% 이하인 셀룰로오스 함유 원료를 분쇄기로 처리하여, 상기 셀룰로오스 I형 결정화도를 33% 이하로 저감하는, 생산성이 뛰어난 비결정화 셀룰로오스의 제조방법에 관한 것이다. 셀룰로오스 I형 결정화도(%)=[(I22.6-I18.5)/I22.6]×100(1) [I22.6은 X선 회절에 있어서의 격자면(002면)(회절각 2θ=22.6°)의 회절강도, 및 I18.5는 비정질부(회절각 2θ=18.5°)의 회절강도를 나타낸다]
