To conduct switching between flow passages in a high-flow rate zone and a low-flow rate zone to monitor leakage abnormality for a gas appliance and a gas supplying facility.

This circuit-breaker is provided with a time setting means 21 for setting a driving period of a flow rate switching means 16 in a low-flow rate; a counting means 22 for counting a drive-start time of the flow rate switching means 16; a time determining means 23 for determining whether a lapse time in the counting means 22 reaches to a time-set value or not; a flow rate determining means 24 for driving the flow rate switching means 16 to close the second flow passage 1 when an output signal of the time determining means 23, and conversion flow rates in the first and second flow rate conversion means 19, 20 are detected to be a prescribed flow rate or less; and an abnormality determining means 25 for measuring a flow rate by the second flow rate detecting means 18 to determine whether a flow rate value therein is a prescribed leakage value or more or not, after the flow rate switching means 16 is driven by the flow rate determining means 24.