A recording/reproducing apparatus within which tape cassettes of different sizes are installable includes a tension regulator arm, pivotably supported by a base plate, and in elastic contact with the magnetic tape withdrawn from the tape cassette, and a tension regulating belt, attached to the tension regulator arm at one end thereof, and in slidable contact with a reel mount to apply a braking force thereto, the braking force being altered according to the position of the reel mount. The braking force applied by the tension regulating belt to the reel mount is less when a smaller sized tape cassette is positioned in the apparatus as compared to when a larger sized tape cassette is positioned therein.