A gas flow rate is measured and therefrom "consumption state" defined by combination of maximum flow rate, increase of total amount and a consumption time of the gas is detected, and then, the consumption state is compared with a "reference consumption condition" representing a predetermined consumption state; and when the "consumption state" exceeds the "reference consumption condition", a gas control means interrupt supply of the gas; and thereafter, after restoration of the gas control means to gas supply state, the actual gas consumption state is measured for a predetermined time counted by timer when the gas flow rate exceeds a predetermined level.