A magnetic tape cartridge is equipped with a cartridge, a single tape reel, a tape leader pin, and a U-shaped clip. The cartridge case is formed by fastening upper and lower cases of synthetic resin together and has a tape leader opening formed in the upper and lower side walls of the upper and lower cases. The single tape reel has magnetic tape wound thereon and is rotatably housed within the cartridge case. The tape leader pin is firmly attached to a leading end of the magnetic tape, and is detachably held in an upright state between the upper and lower cases so that it faces the tape leader opening. The U-shaped clip is attached to walls near the tape leader opening to fasten the upper and lower cases together, whereby the fastening strength of the upper and lower cases is enhanced.