Plural signal beams enter a planar waveguide laser amplifier from different directions to follow different paths therein, which reduces a region not contributing to amplification where no signal beam passes. Plural signal beams follow different paths in the planar waveguide, and a second signal beam utilizes the gain of a portion of the planar waveguide that a first signal beam cannot utilize. By this configuration, a region not contributing to amplification of the signal beams in the planar waveguide laser amplifier can be reduced, causing to efficiently amplify plural signal beams.