A first exposure treatment for irradiating a first photosensitive layer formed on one main surface of a transparent support with a first light thereby to expose the first photosensitive layer and a second exposure treatment for irradiating a second photosensitive layer formed on the other main surface of the transparent support with a second light to expose the second photosensitive layer are performed such that the first light incident on the first photosensitive layer does not substantially reach the second photosensitive layer and the second light incident on the second photosensitive layer does not substantially reach the first photosensitive layer.