PURPOSE: To prevent an error completely by disregarding the inversion of an output of an comparator so as to disregard the inversion of an undesired comparator output due to a saddle if an output of an LPF is within a prescribed range while bridging over a zero point.
CONSTITUTION: An output of a magnetic head 21 is given to a reproducing amplifier 22 and an LPF23, and the peak is converted into a zero cross point by a differentiation device 24 and an output of a comparator (CP) 5 is inverted at that point. On the other hand, an output of an LPF3 is inputted to CPs 11, 12. The output of the CP11 or 12 goes to an H level depending whether the AC level of the output of the LPF3 is larger or smaller than (V 2 -V 1 ) and an L level output is produced at a NOR gate 13. An output of the CP5 is given to a NOR gate 7 together with the output of a NOR gate 13 subjected to time matching by a delay circuit 14 via an edge output circuit 6, the output inversion of the CP5 is disregarded by a timed main filter 9 via an FF8 and a pulse corresponding to the peak of the output of the head 1 is obtained by a waveform shaping circuit 10.