Servo information is written on a magnetic disk at intervals equivalent to 1/N of where the period at the position of a head is controlled. When the frequency characteristic of a servo loop is measured, a servo controller generates a position signal, which indicates deviation of the head from a reference position, using servo information at a first sampling point, calculates an actuator drive current command value, which drives an actuator, on the basis of the position signal, and controls the actuator based upon the current command value. Further, the servo controller executes processing for generating the position signal, but does not calculate the actuator drive current command value, on the basis of servo information at second through Nth sampling points. A frequency characteristic analyzer inserts a disturbance signal having a predetermined frequency into the servo loop, accepts, at double the sampling frequency, the position signal and the actuator drive current generated based upon each item of servo information, obtains the frequency characteristic of the servo loop and outputs the frequency characteristic.