An optical waveguide apparatus having a very simple structure that can modulate a signal light guided through an optical waveguide is provided. A photoresist 13 is applied to an upper side of an SOI film 12, a photoresist mask 14 is formed, and the SOI film in a region that is not covered with the photoresist mask 14 is removed by etching to obtain an optical waveguide 15 having a single-crystal silicon core. Further, a light emitting device capable of irradiating the single-crystal silicon core with a light having a wavelength of 1.1 μm or below is provided on a back surface side of a quartz substrate 20 to provide an optical waveguide apparatus. When the light emitting device 30 does not apply a light, the light guided through the optical waveguide 15 is guided as it is. However, when the light emitting device 30 applies a light to form each pair of an electron and a hole in the irradiated region 16, the light guided through the optical waveguide 15 is absorbed by the pair of an electron and a hole, thereby enabling switching (modulation) for turning ON/OFF an optical signal depending on presence/absence (ON or OFF) of application of the light from the light emitting device 30.