PURPOSE: To discriminate the tone of water leakage easily from noises to detect water leakage by integrating the continuous time of the tone and only when the integrated value exceeds a previously set up decision reference, deciding the tone as water leakage.

CONSTITUTION: A water leakage tone and a noise detected by a sensor 1 are converted into electric signals and inputted to a waveform shaping circuit 2. The circuit 2 amplifies the electric signals from the sensor 1, filters the frequency and discriminates the signals in accordance with the peak values to shape a signal significant for a water leakage signal as a high level signal e.g. and an insignificant signal as a low level signal. A signal duration time integrating circuit 3 integrates the high level time of the input signal and outputs a time integrating signal T0. a water leakage deciding circuit 4 outputs a decision signal D0 for water leakage only when the time integrated by the circuit 3 exceeds the decision reference value.