PURPOSE: To enable the highly accurate measurement of the transmissivity or absorbancy of a specimen, by calculating the ratio of a chopping cycle for equalizing a reference optical signal and a specimen optical signal by allowing reference light and specimen light to be incident to the same light detector by each chopping.

CONSTITUTION: The light of a light source 1 is divided into luminous fluxes of reference light R and specimen light S and both lights are respectively alternately incident on a monochrometer 6 by a sector mirror 4 through choppers 2, 3 and spectrally diffracted monochromatic light is received by a light detector 7. CPU13 controls the choppers 2, 3 so that the reference optical signal and specimen optical signal outputted from said detector 7 become equal and the transmissivity or absorbancy of a specimen is calculated on the basis of the ratio of the chopping cycles of the choppers 2, 3 at this time. By this method, crosstalk is eliminated because the discrimination of lights R, S are unnecessary and a shape error becomes largely effective in an area having a large light shielding rate and highly accurate measurement without an error can be performed.