A flow rate of gas is measured with a flow rate measurement means and a "consumption state" defined with a combination of maximum flow rate, the total amount and a consumption time of the gas is therefor detected. The consumption state is compared with a reference consumption condition which is represented by a predetermined consumption state, and when the consumption state exceeds the reference consumption condition, supply of the gas is interrupted. Furthermore, a monitor range of the consumption state is formed, and when a consumption state is present in the monitor range, the reference consumption condition is changed.