PURPOSE: To enable a misalarm due to a pilot flame to be reduced and reliability for detecting a leakage to be improved by memorizing a gas flow according to a combination of a plurality of gas pilot flame for a certain period for distinguishing these pilot flames from an inner-pipe leakage.
CONSTITUTION: A use state detection portion 2 receives a gas flow signal G which is detected by a flow-measuring means 1, and a timer signal I or a reset signal R1 are output, respectively, depending on whether a pause interval of the gas flow signal G is shorter or longer than a specified long pause period. Also, when the pause interval is longer than a specified short pause period, a count signal C is output. A first timer portion 3 which receives a timer signal T continues counting for a specified period and an alarm signal A is output if a reset signal R1 or R2 is input during this period. On the other hand, a use state operation portion 4 continues the count signal C for a specified number of times, calculates a flow operation data Q in the case of a stable reception, and outputs this data Q to a plurality use state memory portion 7 while a second timer portion 5 is in operation.