One of silicon dioxide particles and an acid whose pKa in water at 25° C. is from 1.9 to 4.5 is mixed in a polymer solution, such that a first liquid may be obtained, and another one is added to the first liquid. Thus a dope is obtained. Otherwise, a solvent, silicon dioxide particles and an acid whose acid dissociation constant (pKa) in water at 25° C. is from 1.9 to 4.5 are mixed, such that a second liquid in which a concentration of said acid is 0.5 wt. % may be obtained. The second liquid is added to a polymer solution, and thus a dope is obtained. A cellulose acylate film produced from each dope can be peeled from a support adequately, and aggregation of the particles are reduced. Therefore, a film whose optical properties are excellent is produced at high speed.