To detect gas leak in gas piping, to surely notify a center of information on the gas leak, and to reduce a cost, resources or the like required for useless exchange of the gas piping.

A sound wave reception element 33-1 receives a sound wave signal S1 generated from a gas leak spot L1. A leak signal processing part 37-1 determines a variation between the received signal and a reception signal in the case of no gas leak. When the variation in some frequency specific region is over a threshold and the state over the threshold continues as long as a prescribed time, it is determined that 'a gas leak exists'. The determination result is transmitted as a sound wave signal S2 through piping 5-1, received by this sound wave transmitter/receiver 8, and transmitted to the center 15 through an NCU 11, and a public network 13.