PURPOSE: To perform a correction without using a special gas and improve measuring precision by generating a spectrogram from the ambient air by use of a system under field correction.

CONSTITUTION: A laser beam is emitted to the ambient air to generate a spectral signal from the air, which is then analyzed by a detecting and analyzing part to generate a first measured spectrogram of the air staying within a chamber in a first gas molecular density level. The second measured spectrogram of the air stayed within the chamber in a second gas molecular density level is generated. A dark current signal spectrogram showing only a dark current signal is generated in an interrupted system. A calculating means generates a background signal spectrogram by use of the dark current signal spectrogram and the first and second spectrograms, and also retrieves a standard spectrogram stored in the memory of the calculating means to generate a gain parameter. The correction is completed by the setting of the operation parameter of the system.