Stretching method of film

Excellent in the longitudinal direction and the width direction of the thickness unevenness and less scratches disadvantage adhesive scratches and transfer flaws by the roll, and an object thereof is to provide a particularly stretching a thermoplastic resin film suitable for use in optical applications. The film at a peripheral speed difference between the front and rear rolls a method of stretching in the longitudinal direction, between the post front roll, and irradiating infrared rays by a condenser heater in each film from the bottom side of the upper and the film of the film, the film the longitudinal irradiation length a, the longitudinal irradiation length of the film surface due to infrared focusing light emitted from the bottom side of the film as a b of the film surface by the focused light of infrared light irradiated from the upper side, a overlapping the parts and part b, and, a, b are the solutions to the longitudinal stretching of the film in both 10mm or 40mm or less.