PURPOSE: To hold a stable state after the start of writing, by starting the writing operation by discriminating not only the range of speed variations but also a variation rate decreasing below prescribed values at the rise of the speed of a magnetic tape.

CONSTITUTION: A comparing circuit 12 makes sure that a period T is less than a T" and greater than a T' at a rise of the speed of a magnetic tape, and also that the variation rate ΔT of the period is less than a prescribed value ΔT', and when both the conditions are satisfied, writing operation is started. Namely, the comparing circuit 12 consists of a comparator 121 and a comparator 122; when the count value T of the period T is greater than the T' and less than the T", the output of the comparator 121 is supplied to a register 11, whose contents are inputted to the comparator 122 together with the contents of a counter 7. Then, its variation rate ΔT is compared with the constant value ΔT' and when ΔT<ΔT", an ON signal is sent to a discriminating circuit 9.