A magnetic detection element is employed. An output voltage from the magnetic detection element is amplified by an amplifying circuit. A switch circuit is connected between the magnetic detection element and the amplifying circuit. The switch circuit reverses the polarity of the output voltage from the magnetic detection element selectively and inputs an output signal to the amplifying circuit. A comparator compares the output signal from the amplifying circuit and a reference value to output a comparison result. First and second storage circuits are provided to receive output signal from the comparator. An electric power control unit controls at least the electric power to be provided to the magnetic detection element. First and second gated signals are provided to the first and second storage circuits respectively. A signal based on the first and second gated signals is supplied to the electric power control unit.