The present invention has the object of offering a laser resonator capable of
maintaining high amplification efficiency even if the thermal lensing effect occurring
in the laser medium varies during operation or over repeated operation and suspension
of the laser device. The laser resonator comprises at least a pair of reflection
portions (planar reflective mirrors 3) provided such as to allow a laser
beam to oscillate therebetween; a laser medium provided on the optical path of
the laser between the pair of reflection portions; an excitation portion (excitation
laser device 5) for exciting the laser medium; an optical system (convex
lens 1) provided on the optical path of the laser beam between the laser
medium and the pair of reflection portions for changing the state of the laser
in the laser medium; and a movement portion for moving the optical system along
the optical axis of the laser.