A self-written branched optical waveguide is formed.A laser beam 2 from a laser source (not shown) is focused with a lens 3 onto the face of incidence 10 of an optical fiber 1. The laser beam of an LP11 mode was emitted from the face of emergence 11, and “bimodal” light intensity peaks were arranged in the horizontal direction (1.A). A slide glass 4 coated with a photocurable resin gel 5 was placed horizontally (1.B). A single linear cured material 61 was formed as the LP11-mode laser beam was emitted from the face of emergence 11 of the optical fiber 1 (1.C). A branch portion 62 was then formed at a distance L from the face of emergence 11 of the optical fiber 1, which was followed by the growth of two cylindrical cured materials 63a and 63b. The two cylindrical cured materials 63a and 63b were linear branches, and formed an angle of about four degrees. An optical waveguide 60 thus formed was composed of cured materials 61, 62, 63a, and 63b (1.D).