Manufacturing method of polymer film

A casting film is made by casting dope including polymer and solvent on a casting drum whose surface is cooled. The casting film becomes to have a self-supporting property by cooling solidification, and then peeled as a wet film from the casting drum. The wet film is fed into a pin tenter in which both side edges of the wet film is held by plural pins. The wet film is dried while conveyed through the pin tenter, to become a film. Next, the film having predetermined X-ray diffraction intensity is fed into a clip tenter. While the film whose both side edges are held by clips is conveyed, it is stretched in the width direction thereof at a stretch rate in a range of 15% to 40%, to control molecular orientation in the film.