Abstract: In an optical system having at least one optical part other than mirrors, for example, a wavelength conversion optical system which uses an external resonator, an optical system which is capable of solving the problem of transverse mode generation due to the existence of scattering matter and the optical crystal used and capable of supplying a stable output (stable wavelength conversion output) is provided. At least at one point in a stable resonator having optical parts (optical crystal) other than mirrors, an aperture having a circular hole with a diameter 1 to 10 times the diameter of a beam at the aperture, a slit having a width 1 to 10 times the diameter of the beam, or an aperture of an arbitrary shape having an edge is located, or a knife edge having an linear or curved edge is Located so that the nearest distance between the edge and the beam is 1 to 10 times the radius of the beam from the center of the beam.