Positive charges of a sensor element are stored in a capacitor for converting a voltage, are converted into a positive voltage by an amplifier, and then are outputted. When the polarity of the charges of the sensor element are inverted to the negative, the charges inversely flow from the capacitor for converting the voltage and feed back to an equivalent capacitor of the sensor element, and an output of the amplifier reduces and returns to a zero-point, an automatic correcting circuit makes an FET conductive and the charges are discharged in order to prevent the fluctuation of the zero-point level due to the negative drift.