The present invention causes measurement light, emitted from an object and to be measured, to enter a fixed mirror and a movable mirror forming interfering light between the measurement light reflected by the fixed mirror and measurement light reflected by the movable mirror. Change to the intensity of the interference light of measurement light is obtained by moving the movable mirror unit, acquiring the interferogram of measurement light. Reference light of a narrow wavelength band included in a wavelength band of the measurement light enters the fixed mirror and the movable mirror, forming interference light of the reference light. The movable mirror is moved to correct the interferogram of measurement light, which is at the same wavelength as the reference light in the measurement light, and the reference light, and a spectrum of the measurement light is acquired based on the corrected interferogram.
