A dispersion-type spectral element designed for reflecting an incident sample light is rotated by a drive mechaism. The sample light reflected by the element is applied to a sample-light receiving device. The device converts the light into a first signal representing the intensity of the sample light. Reference light consisting of beams having various wavelengths over a broad range is supplied from a reference light source to the spectral element. Further, standard light consisting of one beam of a known wavelength is also supplied from a standard light source to the spectral element. The reference light reflected by the element is applied to a reference-light receiving device. The reference-light receiving device converts the reference light into a second signal representing the intensity of the light. An etalon is located in the optical path between the spectral element and the reference-light receiving device. The standard light reflected by the element is applied to the standard-light receiving device. The standard-light receiving device converts the standard light into a third signal representing the intensity of the standard light. An arithmetic unit receives the first, second, and third signals separately, and obtains from the second and third signals a wavelength scale used for calculating from the first signal the wavelength of the sample light.