A magnetic disk apparatus includes a following compensator for measuring a frequency
characteristic of an under-control object to stabilize a vibration mode of the
under-control object with phase condition and a resonance filter having a resonance
point in the vicinity of a frequency at which positioning accuracy is to be enhanced,
being coupled in parallel to the under-control object. Polarity of mode constant
of the resonance filter is so determined as to stabilize a vibration mode brought
about by the resonance filter on the basis of a relation between the frequency
stabilized by a control system and the phase-180 crossover frequency. Positioning
accuracy at a specific frequency can be enhanced without involving unstability
in the control system even in a frequency range where sensitivity function of the
control system is higher than 0 dB inclusive.
