Magnetic tape back tension device for magnetic recording-reproduction system

A magnetic tape back tension device in a magnetic recording-reproduction system having a band brake mechanism for lightly applying a braking force to a supply reel support, the band brake mechanism comprising a brake release lever (42) rotatably mounted on a chassis, a band brake (41) having one end attached to the brake release lever, reeved around the supply reel support and attached at the other end thereof to a tape tension maintaining mechanism (31), and a spring (43) for biasing the brake release lever into rotation in a brake applying direction, and further having a control plate (2) slidably provided on the chassis, the control plate comprising a first contact face (25) movable into contact with the lever biasing spring by the sliding movement of the control plate toward a mode change position to free the brake release lever from the force of the spring, and a second contact face (26) movable into contact with the brake release lever by the sliding movement to rotate the lever in a brake slackening direction.