A magnetic recording tape cassette having a casing and a pair of reels mounted for rotation in the casing for winding a magnetic recording tape therearound is provided with a braking member for preventing rotation of the reels in the unwinding direction while the cassette is not in use. The braking member comprises a body portion movable in the direction perpendicular to the line passing through the center of the reels back and forth with respect to the line between an operative position remote from the line and an inoperative position near the line, a pair of wing-like swinging portions integrally extending from the body portion on opposite sides thereof, and engaging hooks formed on the free end of the respective swinging portions to be engaged with gear teeth formed on the periphery of a flange of the reels. The swinging portions are resiliently urged to an engaging position in which the engaging hooks are brought into engagement with the gear teeth of the respective reels to prevent rotation of the reels in the unwinding direction. The body portion is normally held in the operative position by a counterforce exerted thereon against the resilient force urging the swinging portions through the engagement between the engaging hooks and the gear teeth. A pair of engaging pins are provided on opposite sides of the body portion to engage with the swinging portions to move them to a disengaging position in response to the movement of the body portion to the inoperative position.