A dope is discharged from a casting die toward a belt to form a film-like material above the belt. Thereby a range near the film-like materials is decompressed by first and second decompression chambers. A pipe connecting each decompression chamber to a pressure controller has an inner diameter from 70 mm to 700 mm, and a length of the pipe is at most 30 m. Further the pipe is provided with a first expansion silencer of expansion silencer and a second expansion silencer of resonance silencer, and a sectional dimension ratio of the first expansion silencer to the pipe is from 5 to 500. Thus a pressure fluctuation in the second decompression chamber and the pipe is reduced and the film-like material becomes uniform. Thus the obtained film has no thickness unevenness, and used for producing a polarizing filter excellent in optical properties and a liquid crystal display.