PURPOSE: To prevent the generation of an error pulse by making an outer-side differential waveform closer to an innerside differential waveform by varying the bias point of differentiating operation corresponding to the track position of a magnetic disk.

CONSTITUTION: A signal read from a magnetic disk is inputted to a peak detecting circuit 4 through an amplifier 1, a filter 2, and a differentiating circuit 3' and the bias point of the transistor TR of the circuit 3' is varied by a track counter 5, a D/A coverter 6 and a coil L. Namely, the counter 5 indicates the track position of the magnetic disk from which the information is read, and its count value is converted into an analog voltage by a D/A converter 6. Therefore, the analog voltage is applied to the base of the transistor TR through the coil L, so its base potential varies with the count value of the track counter 5.