To provide a method for manufacturing a cellulose acylate containing few minute foreign matters and suitable as a material for an optical film.

The manufacturing method of the cellulose acylate having substitution degrees A and B satisfying the formulae; 2.0≤A+B≤3, 0≤A≤2.5 and 0.3≤B≤3 (wherein A is a substitution degree of acetyl groups; and B is the total of the substitution degrees of 3-7C acyl groups) comprises (1) a step for adding to a cellulose at least one of water and a 2-7C carboxylic acid as an activating agent, (2) a step for irradiating the cellulose to which the activating agent has been added with microwaves and (3) a step for adding an anhydride of the 2-7C carboxylic acid to the cellulose and acylating hydroxyl groups in the cellulose in the presence of a Broensted acid.