A controller corrects a spectrum S (λ) detected at a wavelength λ of signal light to S′; (λ) in accordance with expressions below: I(λ)=(I2−;I1)×(λ−;λ1)/(λ2−;λ1)−;I1, and S′;(λ)=S(λ)−;I(λ), where I1 is the intensity of infrared light detected at a wavelength λ1 of reference light and I2 is the intensity of infrared light detected at a wavelength λ2 of correction light.