PURPOSE: To obtain a wavelength shifting element having a stable resonator structure by providing a high-refractive-index part having a lenticular action under the surface of a nonlinear optical crystal.

CONSTITUTION: A ring resonator is formed by the three faces of an LiNbO3 crystal 5 as a nonlinear optical crystal. One of the optical paths of the resonator is matched with the Y direction as the orientation where the phases are matched in wavelength shift. A high-refractive-index part 6 having a lenticular action is provided on one face on the optical path of the resonator to stabilize the resonator consisting only of flat faces. A film 7 having a high reflectance for the light of the wavelength is provided on the face into which the fundamental wave is injected and total-reflectance films 8a and 8b having the reflectance close to 100% on the other two faces.