To provide a solution method for producing a film which causes no problem in drying nonuniformity, belt temperature nonuniformity, belt planeness, etc., even when a belt of at least 1.6 m width is used and can improve the productivity of the film.

In the method, the belt at least 1.6 m in width is used as a support for supporting the film of a polymer solution cast from a die, the snaking quantity of the belt is ±0.5% of the width of the belt, and the casting width of the film from the die is adjusted to be 70% of the belt width to the belt width - (the snaking quantity × 4).