The position error signal (PES), representing relative position regarding position information (RRO) previously recorded on a magnetic disk and the magnetic head, is averaged in each servo sector by an averaging unit, and a control input is obtained based on the PES by a phase compensator and the control input undergoes an averaging process in each servo sector by another averaging unit. Convolution integration with respect to the signal data processed by those averaging units is carried out by using a transfer characteristic formed by adding 1 to an open-loop transfer characteristic, which is determined by a magnetic head actuator model and a servo control circuit, to obtain a synchronous vibration estimate. This estimate undergoes a high-pass filtering process of a zero phase error characteristic by the filter unit to remove the vibration component of low frequency band and to thereby obtain a synchronous vibration learned value with high precision.