To easily perform calibration of a flowrate meter by measuring in real time, gas flowrate resistive to disturbance without being affected by the air temperature and pressure variation.

The flowrate of gas supplied to a external gas using equipment 9 through a tube 3 from a gas cylinder 2 of a gas supply system 1 is measured with a flowmeter 4, its measurement result is output to an arithmetic unit 5 where, with the use of the measured data and a measurement data of a balance 6, the opening of a valve 8 is adjusted so that a specific flowrate is supplied. The balance 6 measures the total weight of the gas cylinder 2 to obtain weight variation at a specific time interval, which is input to a comparator 7 by obtaining gas mass flow as a quantity of gas consumption from the gas cylinder 2. The flowmeter 4 is calibrated with the accurate flowrate value input from the balance 6 in the arithmetic unit 5 and obtains calibration data of the flowmeter. By statistically processing the flowrate measurement data of the balance 6 and the flowmeter 4, the opening of the valve 8 is controlled.