PURPOSE: To hardly affect the efficiency of conversion to a higher harmonic wave by the machining deviation by specifying the ratio between the resonator length and the radius of curvature of a spherical mirror for resonance.

CONSTITUTION: The main part of a second harmonic generator 101 is constituted by successively arranging an LED 102 as the light source of a basic wave, coupling optical system 103 composed of a collimator lens and a mode matching lens or the like, and monolithic ring type resonator 104. Then, the resonator 104 provided with a non-linear optical material for converting the fundamental wave to the higher harmonic wave so as to resonate the fundamental wave is the ring type resonator 104 composed of two spherical mirrors for resonance, which are faced each other, and one planar mirror and a ratio (L/R) between a resonator length L and the radius R of curvature of the spherical mirror for resonance is set larger than 1.4. Therefore, even when the center axis of curvature 114 is inclined at α=0.15° to an (a) axis 113, a higher harmonic wave output more than 80% of the case of completely eliminating the machining deviation of the center axis of curvature 114 can be provided at this monolithic ring type resonator 104.