To detect external vibration, impact, or gyroscopic moment exerted on a disk directly with quick response and sufficient sensitivity.

A disk device is provided with a discoid disk which records or reproduces an information signal, and a spindle motor attached to a turntable which rotates the discoid disk. Facing to the bottom end 8' of a rotor magnet of the spindle motor 1, a Hall sensor 11 for detecting the change of magnetic flux is arranged on the upper surface of the mounting base of the spindle motor. A sudden variation of motion to the bottom end of the rotor magnet of the spindle motor during a rotational operation of the spindle motor is detected by means of a magnetic flux change of the Hall sensor 11. Two or more of Hall sensors are attached on the upper surface of the attaching base. The amount of the amplitude variations of magnetic flux obtained from each Hall- sensor output is detected, and the amount of gyroscopic moments exerted on the discoid disk is calculated.