PURPOSE: To run a magnetic tape even at an extremely low speed, by measuring the traveled amount of the magnetic tape per specified unit time, comparing this value with a prescribed value, and applying the result of comparison to a magnetic tape driving means as a pulse width modulation signal.

CONSTITUTION: A DC motor 13 generates a rotary rorque T in the direction of the arrow C when an input signal of a driving circuit 22 is at high level, and does not the torque T when low level. The rotary angle of a capstan 11, i.e., the amount of the magnetic tape 10 is detected with a disc 14 with slits, a light emitting device 15 and a photodetetor 16. The light outputted from the device 15 intermitted with the revolution of the disc 14, and inputted to the photodetector 16. The photodetector 16 outputs a pulse signal P indicating the intermission and the amount of the magnetic tape 10 is measured by counting the pulse signal P.