A belt-like work (11) provided with a photosensitive layer is conveyed in a work conveying direction F at a work conveying speed V. An illuminating section (30) illuminates a photomask (29) in an exposure period T synchronized with the work conveying speed V. The photomask (29) is disposed at a proximity gap from the belt-like work (11). Mask patterns (33) on the photomask (29) are exposed on the belt-like work (11) to form periodic patterns thereon.