PURPOSE: To always hold suitable amplification factor and to stablize an output by controlling the amplifier of a control signal, which is recorded to a magnetic tape, to the amplification factor corresponding to the traveling speed of the tape.

CONSTITUTION: Since a voltage to be generated in a magnetic head 2 is changed corresponding to the speed for this head 2 to pass in a magnetic field, the amplitude of a control signal 9 is changed according to the traveling speed of a magnetic tape 1. When this change of the amplitude is inputted to an amplifier 3, the control of the amplification factor is achieved by an amplification factor controller 4 so that the output can be the fixed amplitude. Then, the amplification factor is controlled corresponding to the traveling speed of the magnetic tape 1. Accordingly, the signal of a frequency proportional to the tape speed to be output from a frequency generator 8, which is attached to a driving means 7 to drive a tape traveling means 6, is converted to a voltage by a frequency - voltage converter 5 and based on the voltage, the suitable amplification factor is produced. Thus, the output signal of the suitable amplitude can be always obtained.