A subtractor subtracts a first control signal output from a first bandpass filter from an error signal, and supplies a differential signal to a second bandpass filter. The second bandpass filter, which has a central frequency of 70 Hz, is affected by an operation of the first bandpass filter, i.e., a first control signal, which has a central frequency of 40 Hz. The first bandpass filter is not affected by an operation of the second bandpass filter, i.e., the second control signal.