To obtain a plane waveguide having optical nonlinearity by using a glass substrate.

A metallic film 12 is formed on the surface of a glass substrate 10 and a pair of electrodes 12a, 12b facing each other via a prescribed spacing are obtd. by etching the metallic film 12. Germanium, etc., are then doped with these electrodes 12a, 12b as a mask to obtain a core part 14. Next, a high voltage is impressed between the electrodes in the state of irradiating the electrodes with the UV rays by which the core part 14 is subjected to UV excited poling and nonlinearity is imparted to the core part 14. The light conducted to the core part 14 is controlled by controlling the impression of the voltage on the core part 14 having the nonlinearity.