Abstract: In an optical parametric oscillator including a laser medium (3) for producing a fundamental wave, a pair of optical elements (4 and 8) constituting a resonator exhibiting resonance for the fundamental wave, and a nonlinear optical crystal (7) between the optical elements for carrying out wavelength-conversion on the fundamental wave to produce a wavelength-converted wave so that one (8) of the optical elements transmits the wavelength-converted wave as an oscillator output wave, a saturable absorber (10) is located between the optical elements. The saturable absorber has a saturable absorption characteristic for the fundamental wave to allow transmission of the fundamental wave of a higher intensity higher than a predetermined intensity and to disallow transmission of the fundamental wave of a lower intensity hot higher than the predetermined intensity.