PURPOSE: To make a device small in size and light in weight by engaging a tension arm with the peripheral face and the step part of a cam in accordance with movement of the cam and setting the tension arm for a magnetic tape to the stretching state in accordance with the engagement state.

CONSTITUTION: When a sliding body 21 is separated from a tension arm 23, the tension arm 23 is turned counterclockwise around a supporting shaft 25 by the elastic force of a spring 27. A link 212 is pressed by a pin 24 provided on the tension arm 23 and is turned clockwise around a supporting shaft 214, and a pin 213 provided on the link 212 is put into a notch 20 provided in a cam 65. When the tensino arm 23 is turned to give tension to a magnetic tape 280, a band brake 28 is wound around the side face of a supply reel base 30 and is pressed there. The tension arm 23 and a sector gear 215 are turned around a supporting shaft 25 independently of each other.