A method of preparing a cellulose acylate of a specific substitution degree, which includes: adding at least one selected from the group consisting of water and a carboxylic acid having 2 to 7 carbon atoms, as an activating agent, to a cellulose; and applying at least one treatment selected from (a) maintaining the cellulose at a temperature of 40° C. or more, for one hour or more, (b) irradiating microwaves to the cellulose, and (c) placing the cellulose under a pressure in the range of from 1.5 atm (0.15 MPa) to 100 atm (10.13 MPa); a cellulose acylate film prepared by means of a solution-casting film formation method or melt-casting film formation method, using the cellulose acylate which is prepared by the preparation method; and a polarizing plate and a liquid crystal display device, each of which contains the cellulose acylate film.