To reduce the pressure loss by preventing the fluctuations of opening of an on-off valve by pulsation, while reducing power consumption.

At the time of regular measurement, the on-off valve 14 is fixed at the full open position 14'. The flow rate of the gas flowing in from an entrance 11 is measured by the ultrasound transducers 26, and 27 of a main measurement part 24. When the flow rate becomes lower than a prescribed value, the on-off valve 14 is closed and a micro flow rate detection part 21 is made to operate. A shut-off valve 17 is open. Gas flowing in a bypass flow path 16 is detected by the ultrasound transducers 19 and 20, and if the flow rate is ≤5L/h, no leakage is determined. When the determination is finished, the transducers 19 and 20 are halted.

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