PURPOSE: To achieve a higher detection accuracy with the stabilization of an output voltage by converting a pulse signal from a signal detection means to a plurality of pulse signals to be inputted into an F/V conversion means.

CONSTITUTION: A pulse signal from a water quantity sensor is converted into two pulse signals with differentiation circuits 75 and 76 of a waveform shaping circuit 74 provided in a water quantity detection circuit 66 and, then, to output voltages corresponding to the number of pulse signals converted with an F/V conversion circuit 72. So, when one pulse signal comes from the water quantity sensor, with the water quantity detection circuit 66, it is converted into two pulse signals and with one pulse signal at an input terminal 71, a charging voltage of a capacitor 77 is raised, thereby permitting a CPU to detect action of the water quantity sensor depending on this higher output voltage.