An optical resonator includes a casing and various optical elements (laser crystal, SHG, etalon, movable mirror) provided within the casing. The optical resonator causes light from the excited laser crystal to resonate and, using the etalon, emits single longitudinal mode laser light. The casing is formed with a low thermal expansion metal exhibiting a thermal expansion coefficient within a range of 0.1 to 3.0×10−6K−1 and thermal conductivity within a range of 10 to 15 W·m−1·K−1. A first temperature control system controlling the temperature of the laser crystal and SHG to be a constant temperature is provided at a placement portion of the laser crystal and SHG. An angle adjuster adjusting an incident angle of the etalon and a second temperature control system controlling the temperature of the etalon to be a constant temperature are provided at a placement portion of the etalon.