PURPOSE: To prevent erroneous judgment due to the insulation failure of an electrode and the failure of a circuit and to achieve fail-safe for preventing an accident by detecting water level according to the difference in characteristics of pulse signals.

CONSTITUTION: An AC voltage is applied to a water level detection electrode 2 and two thresholds V1 and V2 are set between the value of the detection signal of the arrival of water level and that of the detection signal of the non-arrival of water level. A 'True' logic judgment is performed when the detection signal is within the thresholds V1 and V2, the logic judgment output signal is smoothed, and then the smoothing output signal is compared with a specific threshold V3, and then a pulse signal is generated when the threshold V3 is exceeded. The pulse signal becomes a pulsive signal when the water level reaches a water level detection edge 2a of the detection electrode 2 and becomes a non-pulsive signal with a certain value when the waver level does not reach the water level detection edge 2a or when the detection circuit and the water level detection electrode 2 are subjected to insulation failure.