Linear notch patterns at fixed intervals are formed by contacting blades 6a on a drum 6 with a film F and mechanically transferring the shape of the blades 6a to the film F. The notch pattern is formed to be approximately parallel to the axial direction of the drum 6. When a flexural stress is applied in the direction of the film F transport by passing the film over the bending roll 8 while applying a predetermined tension thereto, crazes or cracks are formed inside the notch patterns, with the notches in the notch patterns serving as starting points thereof. Since crazes or cracks are formed using the notch patterns as starting points, relatively large crazes or cracks can be obtained, and optical characteristics can be controlled at a high level.