Method for producing diagonally stretched film

A method for producing a diagonally stretched film includes a diagonally stretching step and a heat fixing step. In the oblique stretching step, the film is stretched in an oblique direction with respect to the broadside direction by holding both ends of the film in the broadside direction with a pair of grippers while relatively advancing the grippers on one side and relatively delaying the grippers on the other side to convey the film. The heat fixing step is a step for fixing the optical axis of the diagonally stretched film after the diagonally stretched step is completed. And a heat fixing step for widening the obliquely stretched film after the end of the obliquely stretching step. When the width of the diagonally stretched film after the diagonally stretched film is completed and before the diagonally stretched film is broadened is L1, and the widths of the portions broadened on the leading side and the retardation side in the thermal fixing step are L2 and L3, respectively, respectively, the relations