PURPOSE: To extract a signal to be measured by one measuring probe by extracting A- and B-head signals by band-pass filters and generating pulses for the switching signal for an analog switch.

CONSTITUTION: An ATF signal outputted by a waveform generating circuit 1 is passed through a BPF 2B to extract only f3 in ATFs at both ends present across a PCM signal, and a monostable multivibrator 3B generates a switching pulse F5 for extracting a B-head component. When data read by an A head is to be measured, the signal is passed through a BPF 2A which passes the signal f1 to extract a signal F1 wherein the A-head and B-head components are both present. At this time, the pulse of the B-head component is inverted to obtain an inhibiting signal, which is inputted to the reset terminal of the monostable multivibrator 3A to cancel the waveform in the period of the inhibiting signal, so that an output F4 becomes a control signal for passing only the A-head component of the analog switch 4A.