A recording and/or reproducing apparatus for a magnetic tape cassette has a back-tension arm pivotably mounted on a mechanical chassis of the apparatus and an actuator mechanism for releasing braking force applied on a pair of tape reel bases by a pair of braking arms while a magnetic tape is run at a high-speed speed. The actuator mechanism includes an axially sliding connector rod engaging with a slider through which said back-tension arm is actuated according to movement of the connector rod, whereby the back-tension arm may apply light friction to the take-up reel base in a reverse running mode in which the magnetic tape is fed from the take-up reel to the supply reel by rotation of a capstan and pinch roller at a relatively slow speed. This causes slack in the magnetic tape to prevent when the tape is started in the reverse running mode.