There is provided an optical parametric oscillator capable of converting a wavelength in a broader range and generating an output beam with high efficiency. The optical parametric oscillator includes: a non-linear optical material optical parametrically converting a beam pumped from a laser; and input and output optical devices opposing each other, the input and output optical devices guiding the optical parametrically-converted beam to the non-linear optical material to oscillate, wherein the input optical device includes an input optical mirror guiding the pumping beam into the oscillator, and the output optical device includes a plurality of output optical mirrors each guiding the optical parametrically-converted beam outside the oscillator, the output optical mirrors having reflectivities different from one another with respect to a wavelength of the optical parametrically-converted beam.