To provide a method of synthesizing laser beams, a laser beam synthesizing device, and a laser beam machining device, with which a hole of a large diameter is machined even with a low output laser oscillator and a working efficiency and a machining quality are improved.

Linearly polarized light beams 2a and 2b of P-wave parallel with each other outputted from laser oscillators 1a and 1b are made incident on beam splitters 9a and 9b which transmit a P-wave of a linearly polarized light beam which is made incident at 45 degree and reflect a S-wave of a linearly polarized light beam. A transmitted beam 2ap passing through the beam splitter 9a is transformed into a beam 2aS of a linearly polarized S-wave whose phase is shifted by 90 degree with respect to the beam 2a by means of a λ/4-plate 13 and a total reflection mirror 14, and is made incident on the point SB of from which a transmitted beam 2bp outgoes. The beam axis of the beam 2aS reflected on the point SB is coaxial with the beam axis of a transmitted beam 2bp, thus the beam energy of a elliptically polarized light beam WA is a sum of the energy of the beam 2aS and the energy of the transmitted beam 2b.