PURPOSE: To obtain a measuring device for discharged gas amount with which the discharged gas amount-time curve has no change, by securing the shift of the orifice plate which possesses the orifice hole to make the gas pass when the amount of gas is measured.

CONSTITUTION: Screen 8 possessing pierced hole 9 is provided between sample room 2 storing sample 1 with the amount of gas is measured and exhaustion room 14 connecting to room 2. Before measurement, lift rod is moved up and down by the function of air cylinder 17, and orifice plate 11 containing orifice hole 10 for passing of gas is lifted up to open hole 9. And at the time of measurement, hole 9 is blocked with exhaustion, and thus no change is given to the gas discharge amount-time curve. Accordingly, the measurement pressure of vacuum gauges 23 and 24 is measured between with sample 1 added and with no sample added, thus ensuring the high-accuracy measurement for the gas amount discharged from sample 1 only.