PURPOSE: To ensure the high-accuracy measurement for the discharged gas amount of various materials by providing the variable orifice between the sample room and the exhaustion tube and then varying the orifice conductance in accordance with the change of the discharged gas amount.

CONSTITUTION: The device consists of sample room 1 storing sample 9, exhaustion device 27 which adsorbs the discharged gas and exhaustion tube 23 linking room 1 to device 27. The area between room 1 and tube 23 is partitioned via the variable orifice. With turning of handle 18, conductance variable rod 13 receives the re ciprocal motion. And conical valve 12 is put in and taken out the orifice to vary the orifice conductance C, and the shift amount or rod 13 is detected via reciprocal quantity detector 20. As a result, the conductance is varied according to the change of the discharged gas amount of sample 9. Then the discharged gas is exhausted so that the voltage difference within room 1 and tube 23 may be within the range not to cause the build-up. Thus the measurement is possible with a high accuracy.