A magnetic tape cartridge has a case including a datum hole into which a positioning pin is inserted at a predetermined position thereof; a reel rotatably supported on the case with a magnetic tape wound thereon and including an engaging gear formed in the outer periphery of the flange thereof; a rotation restricting mechanism disposed in the case and engaging with the engaging gear of the reel to thereby restrict the rotation of the reel; and, an engagement releasing element slidably inserted into the datum hole from inside the case and removing the engagement between the rotation restricting mechanism and the engaging gear through the sliding movement thereof within the datum hole, wherein the positioning pin is inserted into the datum hole from outside of the case and push and slide the engagement releasing element, thereby releasing the rotation restriction of the reel by the rotation restricting mechanism.