In a Fourier-transform spectroscopy apparatus, a scanning mirror is arranged on a light path of scanning light. The scanning mirror delays or advances the scanning light with respect to reference light according to the rotational angle of the scanning mirror from its initial position. A spectroscopic spectrum generating unit generates an interferogram based on the intensity of the detection target light obtained from the detection target, and Fourier transforms the interferogram thus generated. The spectroscopic spectrum generating unit corrects the nonlinearity of the group delay between an envelope of the reference light and an envelope of the scanning light, and corrects the nonlinearity of the phase shift between the respective envelopes.