Abstract: A method of stretching a film in a longitudinal direction thereof by a difference in circumferential speed between front and rear rolls comprising radiating infrared rays onto the film from above the film and below the film with photo-concentration heaters wherein a represents an irradiated-area length in the longitudinal direction of an area of the film irradiated with converged light of the infrared rays radiated from above the film and b represents an irradiated-area length in the longitudinal direction of an area of the film irradiated with converged light of the infrared rays radiated from below the film, the area having the length a and the area having the length b overlap with each other, and a and b are each 10 mm or more to 40 mm or less.