To obtain a small and compact two frequency laser beam oscillator by creating two optical resonator lengths using a birefringence material arranged in a laser resonator and oscillating the laser beam with two frequencies corresponding to the resonator lengths.

A reflector 1 reflects an incident laser beam in a laser resonator. A solid state laser medium 2 is pumped by a pumping means and oscillated. A birefringence material 3 creates two optical resonator lengths in the laser resonator. The light propagates through a crystal while being separated into two intrinsic polarized lights involving characteristic refractive indexes. In the case of a material exhibiting only birefringence, the intrinsic polarized beams are orthogonal polarized beams involving respective refractive indexes. When such a birefringence material is arranged in a laser resonator, laser beam can be oscillated with two frequencies.