A spectral element separates first laser light in such a manner that an area including only signal light as reflected first laser light is formed on a photodetector. The photodetector is provided with a first sensor group which is disposed at an irradiation position of signal light of the separated first laser light, and a second sensor group which receives zero-th order diffraction light of a main beam and two sub beams of second laser light that has been transmitted through the spectral element. The second sensor group includes a four-divided sensor which receives the main beam. The four-divided sensor is disposed in an area surrounded by the first sensor group. Zero-th order diffraction light of the first laser light that has been transmitted through the spectral element is irradiated onto the four-divided sensor.