To accurately detect leakage of a detection object from a pipe.

In the period when a gas table 5 and a hot water supply machine 6 are not operated, in radio machines 1, 2 provided on one and other of pipes 8 where gas flows, the gas flow in one and the other pipes is detected by the respective corresponding meters 3. The radio machine 1 compares one detected flow with another flow received from the radio machine 2. As a result of comparison, when both match, it is detected that the gas flows without leakage, and when they do not match, leakage can be detected. Accordingly, even if a fuel battery 7 supplied with gas from the pipe 8 on the upstream side or the other downstream side and always operated is connected, the presence or absence can be accurately detected.