To provide a disc-rotating device which can decrease the number of times for switching the brake mode and can make a rotating disc stop quickly.

Based on a brake command signal (S2) outputted from a motor control command generating section 10, the time duration of one pulse of a rotation pulse signal (S1) outputted from a rotation period measuring section 9 and a brake processing period signal (S3) outputted from a brake processing period generating section 11; switching ratio between short brake for reducing rotation of a spindle 3 and reverse brake for reducing rotation of the spindle by rotating the spindle 3 in reverse is determined. Assuming the duty ratio d(Tb/Ta) of a reverse brake action period (Tb) to a brake processing period (Ta), a brake switching signal generating section 12 supplies a brake-switching signal (S4), corresponding to the duty ratio d to the motor control command generating section 10, from which the brake command signal is outputted.