Provided is a small, highly accurate Fourier spectrometer which enables highly accurate detection of an optical path difference in an interferometer. An element for changing to a narrow band is provided to return reflected light to a second light source (4), and the wavelength of light emitted by the second light source is locked, whereby the position of a movable mirror (8) is measured highly accurately and an optical path length (1) and an optical path length (2) match highly accurately.