A laser device 1 demultiplexes seed light L0 into a plurality of beams of laser light L1 and then continuously optically amplifies the plurality of beams of the laser light L1 with an amplifier 14. Therefore, its amplification factor can be set higher than that in the case of amplifying pulsed laser light. When producing multiplexed light L3 by multiplexing the beams of the amplified laser light L1 with a diffraction grating 16, respective phases of the beams of the laser light L1 are controlled such that an output peak of the multiplexed light L3 repeatedly appears at a converging position P1 at a predetermined time interval. This produces pulsed laser light at the converging position P1 from a plurality of beams of laser light L2 amplified at a high amplification factor. Hence, this laser device 1 can produce pulsed laser light with a high output.