Provided is a method for preparing a cellulose derivative having solubility improved and therefore having less undissolved floating portions when the derivative is added into water. More specifically, provided is a method for preparing a cellulose derivative, comprising a step of depolymerizing a cellulose derivative to produce a depolymerized cellulose derivative having a viscosity at 20° C. in a 2% by weight aqueous solution of the depolymerized cellulose derivative reduced by at least 10% compared with that of the cellulose derivative before the depolymerization so that the number of undissolved floating portions in the aqueous solution of the depolymerized cellulose derivative is decreased compared with that of the cellulose derivative before the depolymerization. Depolymerization is effected preferably by an acid, alkali or enzyme.