PURPOSE: To provide a method for measuring the quantity of gas discharged from a sample, e.g. a semiconductor wafer, within a unit time in which the quantity of gas discharged from the sample can be dealt with flexibly over a wide range.

CONSTITUTION: A second orifice 30 having freely variable conductance C2 is disposed in a second vacuum chamber 21-2 interposed between a first orifice 22 and a second vacuum gauge 25-2. Quantity Q of gas discharged from a sample 11 within a unit time when the sample 11 is heated by a heating means 27 in the first vacuum chamber 21-1 is determined based on the difference of the degree of vacuum P1-P2 between the first and second vacuum chambers 21-1 and 21-2, and the product C3×(C1-C2) of the difference (C1-C2) between the conductances C1 and C2 and an effective conductance C3.