PURPOSE: To measure the component for measurement accurately, by eliminating interference light other than Raman scattered light, measuring the total Raman scattered light intensity and the Raman scattered light intensity of the component for measurement, and determining the ratio of the two.

CONSTITUTION: A laser beam leaving a laser beam source 1 radiates a specimen 2, from which scattered light and reflected light are generated, which, then, enter a sharp cut filter 5, and only Raman scattered light is transmitted. The Raman scattered light enters a mirror of a known transmittance, which reflects a majority thereof while transmitting only a part, so that a part of the Raman scattered light is led into a photo detector 7 while the rest is reflected and reaches a detector 10 after spectral analysis at a mirror 8 and a diffraction grating 9. Signals from detectors 7 and 10 are fed into a division circuit 11, and the Raman scattered light intensity A of the component being measured as measured in the detector 10 is divided by the total Raman scattered light intensity B as measured in the detector 7, and the concentration of the component being measured is calculated in a multiplication circuit 12.