A primary dope is obtained by mixing a solvent and a polymer, and an additive is added to the primary dope, and thus a second dope for a main layer, a first dope for a contact layer and a third dope for an exposure layer are prepared. Only in the second dope is contained at least one of retardation controller, UV absorbing agent and deterioration inhibitor. The first-third dopes are cast from a casting die onto a casting belt to form a casting film of a three layer structure having the base, contact and exposure layers. Since the volatile compound is contained in the main layer, a precipitation of the volatile compounds on a surface of a produced film is prevented.