According to the present invention, a film is formed from a powdered cellulose acylate resin using a twin-screw extruder which has complete-mating-type screws and whose L/D is set in the range of 20 to 55, while setting the temperature of the resin in the extruder in the range of Tm+10.degree. C. to Tm+70.degree. C., keeping the average residence time of the cellulose acylate resin within 5 minutes, and drawing a vacuum so that the degree of vacuum in extruder after the powdered cellulose acylate resin has been melted is kept at 100 Torr or lower, whereby the color change or deterioration in mechanical strength of the resin can be decreased, and thus, a high-quality and high-functionality film which is good for the optical application can be provided.