Patent ID: 7360733

Claim:
A magnetic tape cartridge for a single reel comprising: a case including a datum hole formed by a cylindrical rib having openings only in upper and lower ends into which a positioning pin is inserted at a predetermined position thereof; a single reel rotatably supported on the case with a magnetic tape wound thereon and including an engaging gear formed in the outer periphery of a 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 disposed on a recording and reproducing apparatus, and is inserted into the datum hole from outside of the case to thereby project the engagement releasing element from the datum hole and to position the magnetic tape cartridge at a predetermined position within the recording and reproducing apparatus, and pushes and slides the engagement releasing element, thereby releasing the rotation restriction of the reel by the rotation restricting mechanism; and wherein the engagement releasing element closes the datum hole when said engagement releasing element is slidably inserted into the datum hole.