PURPOSE: To measure the concentration level of an UF6 gas in a short time without sampling, by irradiating uranium hexa-fluoride (UF6 gas) in a piping with laser lights with two different wavelengths to measure the intensity of the individual laser lights.

CONSTITUTION: A UF6 gas is kept flowing through a piping 1. Laser lights with different wavelengths are emitted sequentially with a control computing unit 8 from light emitting bodies 4 and 5 and laser light reception intensity signals from light receiving bodies 6 and 7 provided facing the light emitting bodies 4 and 5 are read into the unit 8. Then, absorptivity curves are calculated for respective wavelengths in the UF6 gas. Thus, the concentration level of the UF6 gas in the piping 1 can be measured in a short time without sampling from the absorptivity curves and the isotope shift wavelengths of UF6.