Laser processing device in which a laser light does not return to the source of the laser light

A laser processing device of the present invention is configured to simultaneously process from both sides of a bonded substrate, in which a laser light does not return to the source of the laser light. A laser processing device 1, which is a device for simultaneously processing from both sides of a bonded substrate W, comprises a laser oscillator 15, a polarization beam splitter 37, a first optical system 5B, and a second optical system 5C. The polarization beam splitter 37 divides a laser light L from the laser oscillator into a first branch light L1 and a second branch light L2. The polarization beam splitter 37 rotates the polarization direction of the second branch light L2 at a specific angle with respect to the first branch light L1. The first optical system 5B irradiates the first branch light L1 on the first surface of the substrate W. The second optical system 5C radiates the second branch light L2 to a second surface of the substrate W.