PURPOSE: To surely execute the read/write operation of data to a storage medium to always execute a rotary movement at a stationary rotating speed by outputting a sure INDEX signal when the spindle mechanism of a data recording and reproducing device reaches a stationary rotary condition.

CONSTITUTION: A motor-on signal to activate a motor 13 is outputted from a control circuit FDMC 24 of a floppy disk device FDD 22, and the driving force of the motor 13 is transmitted through a belt 12 to a pulley 11 of the spindle mechanism. Thus, a slit plate 20 is rotatingly driven, and a slit 20a for a rotation control and a slit 20b for an index detection formed to a plate 20 are detected by a photosensor 21. The photosensor 21 outputs an FG signal according to the detection of the slit 20a, the pulley 11 of the spindle mechanism is gradually accelerated from a low speed, and it is made to reach a constant speed drive. At such a time, the FG signal is changed in a prescribed period, the fact that the period reaches a set value is decided by a pulse period deciding part 23, and the INDEX signal is outputted.