A magnetic tape cartridge includes a cartridge casing and a single reel which is contained in the cartridge casing for rotation. A brake member of the magnetic cartridge is provided with a brake gear and is movable up and down between an operative position and a retracted position. The reel is provided with a reel gear which is brought into mesh with a drive gear of a tape drive mechanism and a through hole through which a push rod extending from a brake release member projects to be brought into abutment against the drive gear of the tape drive mechanism so that the brake release member moves the brake member to the retracted position. The through hole opens in an area which is slightly wider than tips of teeth of the reel gear, wherein the area includes a pair of opposed flanks of the reel gear which form a valley.