PURPOSE: To measure a chemical species precisely for a long time by calculating an absorptivity of laser light due to measuring chemical species, and correcting a measured quantity of fluorescence due to laser excitation, on the basis of absorptivity, i.e. wavelength variation of the laser light.

CONSTITUTION: The laser light 2 emitted from a laser 1 is split by a beam splitter 3, a part of the light 2 is entered a plasma utilizing apparatus 4, and the fluorescence 6 generated by exciting a chemical species of measuring object is condensed at a fluorescence detector 8. Another laser light is chopped by a chopper 12, and the laser light transmitted through a half mirror 9 passes through a cell 11 contg. the chemical species of measring object. A piezoelectric converter 13 converts a sound pressure generated by the laser beam absorbed in the chemical species in the cell 11 into an electrical signal proportional to the absorbed quantity of light. An absorptivity arithmetic device 14 outputs a signal proportional to the absorptivity by the signal sent from a photoelectric conerter 10 and the converter 13, and an arithmetic device 15 of fluorrscene generation rate outputs the signal proportional to the fluorescence generation rate.