Using the dope containing the following cellulose acetate (1), (2), or (3), a film is prepared by the casting process: (1) a cellulose acetate having carboxyl groups binding to at least one member selected from the group consisting of a cellulose acetate and a hemicellulose acetate, wherein at least a part of said carboxyl groups are in an acidic form; (2) a cellulose acetate containing at least one member selected from the group consisting of an acid having an acid dissociation exponent pKa of 1.93 to 4.50 in water, an alkali metal salt of said acid and an alkaline earth metal of said acid; or (3) a cellulose acetate containing an alkali metal or an alkaline earth metal wherein the total content of an alkaline metal and an alkaline earth metal in 1 gram of the cellulose acetate is from an effective amount to 5.5.times.10.sup.-6 equivalent (in terms of ion equivalent). The above cellulose acetate is also useful for spinning process. The cellulose acetate includes a cellulose diacetate and a cellulose triacetate. The cellulose acetate of the present invention has high film-releasability from a support and excellent optical characteristics.