The wavelength detector includes a diffusion element that diffuses the laser beam; a light collection optical system provided downstream from the diffusion element; a member, including an aperture, provided downstream from the light collection optical system; a discharge tube that is provided downstream from the member and that includes a cylindrical anode and a cylindrical cathode that each have a through-hole formed therein, and that is configured so that an electrical property between the anode and the cathode changes due to an opto-galvanic effect when a laser beam having a predetermined wavelength passes through the through-hole of the cathode in a state in which a DC voltage is applied to the anode; and a high-voltage DC power source. The discharge tube is disposed so that the laser beam that passes through the aperture passes through the through-hole of the cathode of the discharge tube without directly irradiating the cathode.