An eddy current sensor has an exciting coil and a detection coil. A holding circuit holds reference data indicating a characteristic of an output signal output from the detection coil at a reference state and outputs the reference data at a state other than the reference state. A pseudo signal generating circuit generates and outputs a balance coil pseudo signal corresponding to the output signal output from the detection coil at the reference state from the reference data output from the holding circuit. A bridge circuit, at the state other than the reference state, receives the output signal output from the detection coil and the balance coil pseudo signal and outputs a bridge output signal corresponding to a difference between the output signal and the balance coil pseudo signal as a bridge output signal.