In a joining section there are columnar distribution pins, on whose periphery a groove is formed. A size of the groove is strictly determined. A first dope and a third dope are fed through the respective grooves such that a width of each dope is controlled. Thereafter, the surface and third dopes are joined with an second dope. Thus a multi-layer stream is obtained and cast through a die lip of a casting die for forming a casting film having a multi-layer structure. The casting film is dried and thus a multi-layer film is obtained.