A method of forming a mirror in a waveguide, including the steps of forming a layer constituting a waveguide in a substrate and forming a mirror-finished surface at a boundary between a portion irradiated with radiation and a non-irradiated portion by obliquely irradiating a layer constituting the waveguide with radiation. The waveguide may be composed of a photosensitive material, and either the irradiated area on the non-irradiated area may be removed by solvent. The waveguide may be composed of a refractive index imaging material in which a refractive index distribution is formed by irradiation with radiation or the refractive index may be increased by heating the irradiated portion. The refractive index imaging material may include a catalyzer to assist in increasing the refractive index distribution of the irradiated area.