PURPOSE: To read the P-P value of a sense signal securely by detecting the 1st peak phase which is in the range of a reading range control signal and then by sending a control signal to a DC regenerating circuit.

CONSTITUTION: When reading range contrl signal RC is at a high levels AND gate 6 is opened and when the 1st peaks P1 and P3 of a sense signal are detected by peak phase detector 5, flip-flop FF7 is set. The output of FF7 is applied to DC regenerating circuit 2 and a DC-regenerated signal is outputted and applied to comparator 3. Once the DC-regenerated signal exceeds the threshold level of comparator 3, its excessive part appears at the output terminal. When control signal RC rises, FF7 is reset to its original state.