Film-stretching apparatus and method of producing film

A method of producing a film, the method comprising:

• a film stretching step of stretching a film,
• the film stretching step being carried out by use of a film-stretching apparatus which includes a stretching furnace having an upstream side including (i) an entrance through which the film is transferred into the stretching furnace, and a downstream side including (ii) an exit through which the film is transferred out of the stretching furnace, the stretching furnace being divided, along a direction in which the film is transferred, into a plurality of air amount control zones,
• wherein an amount of air in each of the plurality of air amount control zones is independently controllable,
• wherein a number of the plurality of air amount control zones is 2n or 2n+1 (n is a natural number),
• wherein each air amount control zone includes an air discharge duct connected thereto,
• wherein, during the film stretching step, a total amount of air to be discharged outside the stretching furnace from the air discharge ducts of n air amount control zones located on the upstream side of the stretching furnace is controlled so as to be larger than a total amount of air to be discharged outside the stretching furnace from the air discharge ducts of n air amount control zones located on the downstream side of the stretching furnace, and
• wherein an airflow in the entrance is controlled so as to be directed only toward an inside of the stretching furnace, such that a volatile component having been volatilized from the film is prevented from leaking out of the stretching furnace through the entrance.