PURPOSE: To prevent a magnetic tape from being stretched permanently, by switching the set value of the tension of the tape from a large value to a small one when the driving load current of a cylinder motor goes higher than a prescribed value.

CONSTITUTION: The tension of a magnetic tape 3 brought into contact slidably with a cylinder 6, and the frictional force of the above stated magnetic tape 3 against a rotary cylinder 6A are increased by the rise of an environmental humidity. And when a driving current I for a cylinder motor 6C which rotates the rotary cylinder 6A exceeds a prescribed value (η), the fact is detected by a detection control circuit 27, and a voltage supply to an energizing solenoid 25 is canceled, and as a result, a spring 21B compresses by a length of (λ). At such a time, so that a control circuit 24 controls a reel motor 1A to keep a light emitting quantity from a light emitting element 22 to a light receiving element 23 at a prescribed value by an arm 21, the length of the stretching length of the spring 25B is held at constant level, and the tension of the tape 3 wound on a back tension pole 11 goes to the prescribed value by being bal anced with a force actuated on the arm 21.