A magnetic tape cartridge includes a cartridge casing formed by upper and lower casing halves connected together, and a single reel around which a magnetic tape with a leader pin fixed to the leading end of the magnetic tape is wound and which is contained in the cartridge casing for rotation. The leader pin is removably held in a tape draw-out opening with its axial direction directed in the vertical direction under urging force of a spring member acting on its upper and lower end portions sideways with the upper and lower end portions received in recesses respectively formed in the upper and lower casing halves. An inclined surface directed obliquely downward is formed at least a part of the side wall of the recess formed in the upper casing half opposed to the direction in which the spring member urges the leader pin.