To directly and accurately measure not only an oxygen quantity in inert gas but also nitrogen and other impurities in it.

Inert gas from an inert gas supplying source passes through a selector valve 3, and then it passes through the first opening and closing valve 6 or the second opening and closing valve 7 to be suplied into a welding chamber 8 from a welding torch 9. An impurity content in the inert gas is analyzed by a quadrupole mass spectrometer 13, and an abnormal signal is output to a welding controller when the impurity content exceeds a fixed level. Although an analyzed result therein may be probably different from an actual impurity quantity in the chamber 8 bacause the inert gas introduced into the spectrometer 13 is very small, preliminary measurement by mixing standard gas spotlikely from a standard gas cylinder 16 is carried out to find preliminarily a measuring critical value.