To uniformize the thickness of a film by good drying in a solution coasting film forming method.

A casting dope 12 containing a polymer, of which the main component is cellulose acylate, is cast on a band 31 to form a cast film 35 according to the solution casting film forming method. A plurality of drying chambers are installed in opposed relation to the cast film 35 and one of two arbitrary drying chambers on the upstream side is set to the first drying chamber 51 and the other one of them on the downstream side is set to the second drying chamber 52 to volatilize the solvent of the cast film. The temperature difference between the air supplied to the first drying chamber 51 and the exhaust gas therefrom is made larger than that of the second drying chamber 52. Further, the exhaust gas of the first drying chamber 51 is raised in temperature by a heat exchanger 57 to be reduced in the content of the solvent and this exhaust gas is used as the air supply of the second drying chamber 52. The obtained film 36 is excellent in the uniformity of thickness and suitable as the optical polymer film. A polarizing plate, its protecting film, an optical functional film and a liquid crystal display device constituted of this optical polymer film are excellent in optical characteristics.