PURPOSE: To obtain a method and an apparatus for measuring the quantity of impurity in a specific gas quantitatively at an ultrahigh purity level.

CONSTITUTION: After an inactive gas of ultrahigh purity is supplied into a sample tube 14 through a purifier 1 (first gas supply source), baking is applied to the inside of the sample tube 14 by heating means 22 to 24. Then, an atmosphere of a prescribed temperature is put inside the sample tube 14 and a specific gas in a prescribed quantity is made to flow into the sample tube 14 by switching first and second selector valves 9 and 10 and 16 and 17. Thereafter the specific gas remaining in the sample tube 14 is expelled, the first and second selector valves 9 and 10 and 16 and 17 are switched, baking is applied to the inside of the sample tube, so as to desorb impurity, and the quantity of desorption thereof is measured.