PURPOSE: To reduce the size of a circuit by varying the cut-off frequency of both an antialiasing filter and a smoothing filter constituting a main filter.

CONSTITUTION: This device is equipped with a phase difference detecting circuit 14 which detects a mutual phase difference after receiving the output of a smoothing filter 13 of each system CH1 and CH2, and a D/A converter 15 which switches the cut-off frequency of a filter 11 to the cut-off frequency of a smoothing filter 13 in order to eliminate the mutual phase difference after receiving the output of the phase difference detecting circuit 14. Thus, the time deviation of the phase difference of a signal waveform passing each channel can be prevented, and the size of the circuit can be reduced.