A frequency shift correcting unit (25) which corrects a frequency shift of a plurality of first signal spectra within the same time range with respect to a frequency of first laser light beam and corrects a frequency shift of a plurality of second signal spectra within the same time range with respect to a frequency of second laser light beam, and a spectrum integrating unit (26) which integrates a plurality of first signal spectra corrected by the frequency shift correcting unit (25) and integrates a plurality of second signal spectra corrected by the frequency shift correcting unit (25) are provided, and a molecular concentration calculating unit (27) calculates a concentration of molecules in the atmosphere from the first and second signal spectra integrated by the spectrum calculating unit (26).