A second operational amplifier (11) of a core (1) has an inverting input terminal and an output terminal that are short-circuited. A signal line (19) is connected to the noninverting input terminal of the second operational amplifier. A capacitance sensor (18) is connected to the signal line (19). A first operational amplifier (12) has a noninverting input terminal grounded. The inverting input terminal of the first operational amplifier is connected to first and second resistors (15, 16) on one end. The other end of the first resistor (15) is connected to an AC voltage generator (14). The other end of the second resistor (16) is connected to the output terminal of the first operational amplifier (11). The core (1) has a signal output terminal (21) connected to an inverting amplifier (2). An addition device (3) is connected to an AC output terminal (22) of the core (1) and the inverted output terminal (42) of the inverting amplifier (2). The noninverting input terminals of the operational amplifiers (36, 40) is grounded.