PURPOSE: To prevent deterioration in efficiency by detecting a servo signal when a step advance is made under the command of a high priority device and finding the movement extent of a capstance motor, and calculating the displacement of a head from a stripe to which the step advance is made and making corrections.

CONSTITUTION: A timing generating circuit 30 generates timing pulses for fetching the movement extent of the capstan motor 31 up to when a trailing servomechanism is read from a start pulse arriving at a terminal B and sends them to a counter 21. The counter 21 counts the rotation extent of the capsten motor 31 and sends the counted value to a control part 20. The control part 20 calculates the rotation extent of the capstan motor 31 by using tachometer pulses from a capstan tachometer detecting circuit 24 in combination to correct a reference value, and sends the corrected value to a write inhibition command generating circuit 28. Thus, the reference value is corrected on the basis of the actual movement extent of a tape to evade an excessive write inhibiting command, preventing deterioration in efficiency.