Cellulose with a calcium (Ca) content of at most 10 ppm is used as a raw material of cellulose acylate. The cellulose acylate is synthesized by esterification of cellulose with carboxylic acid such as acetic acid. A dope containing the cellulose acylate and a solvent is continuously cast onto a belt, which is a support, to form a casting film. The casting film is peeled off from the belt as a film, and then the film is dried.