PURPOSE: To optimally keep tension of magnetic tape immediately after the tape is loaded on a rotary drum by detecting a tape winding diameter of a supply reel during the loading and calculating optimum tension afterward at the time of reproducing the tape.

CONSTITUTION: Rotating speed of the supply reel 1 is detected by a reel rotating speed detecting means 7, and a loading state of the magnetic tape 3 on the rotary drum 6 by guide rollers 4 and 5 is detected by a loading state detecting means 8. Then, a winding diameter of the supply reel is calculated by a supply reel winding diameter calculating means 9 based on the rotating speed detected by the speed detecting means 7 at the time of loading and also at every interval of a prescribed time during the tape travel. Then, driving torque of a motor 13 for driving the supply reel 1 is controlled by a motor driving torque control means 10 in accordance with the winding diameter of the supply reel 1 so as to obtain optimum tension.