PURPOSE: To decrease loss of optical guiding in the stage of providing a mask on a photosetting material layer, irradiating an active ray thereto and forming an optical branch waveguide thereon by disposing an auxiliary mask provided with a specific light shielding surface on the mask and smoothing and boundary surface between the optical waveguide and clad in the branching part.

CONSTITUTION: A photosetting material layer 6 is formed on a base plate 7 and a mask 3 which has a light transmittable surface 5 made to the same shape as the shape of an optical waveguide 1 and is formed with a light shielding surface 4 is superposed thereon. An auxiliary mask 8 is provided via a heat-resistant glass plate 11 on the mask 3. A light shielding surface 10 along the boundary between the surface 5 and the surface 4 is formed to the mask 8 within the surface 5 on the inside angle side of the branching part 9 of the surface 5 formed to the mask 3. Then the width in the shielding part at the actuate angle of the part 9 is made smaller than the width when only the mask 3 is used and the surface of the waveguide is smoothly cured if the layer 6 is cured by irradiating light as shown by arrows. The waveguide is thus sharply formed and the optical transmission loss is decreased.