PURPOSE: To automatically and with high accuracy execute an index adjustment by constituting the titled device so that an adjusting data calculating means calculates an index adjusting data by comparing an index position accuracy data and the result of detection of the leading of an index pulse, and based on the result of calculation, the leading position of the index pulse is adjusted.

CONSTITUTION: The leading position of a detecting pulse from an index detector 15 is delayed by a prescribed time by a timer 16, and an index pulse A is outputted to a CPU 21. The CPU 21 calculates an index position accuracy Q from the pulse A and a burst signal B from an index adjusting disk 17, compares the position accuracy Q and the reference index position accuracy, and outputs an index adjusting data ID for setting an error to '0'. A writing circuit 22 a allows the ID to be stored in a P-ROM 13, and a CPU 14 sets an initialized value corresponding to its ID to the timer 16.