A magnetic tape cartridge includes a cartridge casing and a single reel which is contained in the cartridge casing for rotation. The reel has a reel hub in the form of a cylindrical member which has a bottom wall and is provided with an opening and a stopper gear formed on the bottom wall. A brake member is provided with a brake gear and is movable up and down between an operative position where the brake gear is brought into engagement with the stopper gear to prevent rotation of the reel and a retracted position where the brake gear is disengaged from the stopper gear to permit rotation of the reel. The brake member is normally urged toward the operative position and moved to the retracted position by a pusher member of a tape drive system which acts on the brake member through the opening in the reel hub when the magnetic tape cartridge is loaded in the tape drive system. A continuous wall portion is erected around the opening in the reel hub, and the brake member includes a horizontal portion and a continuous skirt portion which extends downward from the horizontal portion and is in contact with the continuous wall portion of the reel hub to close the opening together with the horizontal portion. The brake member is moved between the operative position and the retracted position with the skirt portion kept in contact with the continuous wall portion of the reel hub.