Optical axes of lens units of measurement heads are adjusted so as to be parallel to each other. The lens unit includes a diffraction lens. When adjusting the optical axis, in a state where the measurement heads face each other with a reference member interposed therebetween, light having a plurality of wavelengths are emitted from the measurement heads to one surface and the other surface of the reference member, respectively. Intensities of primary light having one wavelength, which are reflected by one surface and the other surface of the reference member respectively, and incident on the measurement heads through each of a path of a multi-order light having other wavelengths are displayed on a main display unit as information indicating a degree of orthogonality of the optical axes of the measurement heads with respect to the one surface and the other surface of the reference member.