An optical wiring substrate fabrication method capable of simple formation, by maskless exposure, of an inclined face shape at an end portion of a core layer structuring an optical waveguide. Using an exposure apparatus, image exposure is carried out with a light beam which is modulated by a spatial modulation element in accordance with image information. A predetermined area of a photosensitive material (a photoresist), which is coated on the core layer which is a material of the optical wiring substrate, is exposed by a light beam (UV) and patterned to form an etching mask. A region corresponding to the inclined face, which is to be formed at the end portion of the core layer, is exposed and patterned by the light beam, exposure amounts of which are controlled in accordance with the inclined form of the inclined face, such that an end portion of the etching mask has an inclined face structure. When the core layer is worked by etching using this etching mask, working of the core layer at the end portion of the core layer progresses in proportion to film thickness of the etching mask, and the inclined face is formed.