Disclosed herein is a magnetic tape cartridge including a cartridge case. In the cartridge case, a single reel with magnetic tape wound thereon is rotatably housed, and a leader pin coupled with a leading end of the magnetic tape is caught by a pin catching member of a recording-reproducing unit and is inserted into a tape leader opening formed in the cartridge case. The magnetic tape cartridge also includes a pin retaining spring member which elastically engages the leader pin so that the leader pin is detachably retained at a predetermined position within the cartridge case. Furthermore, a wall surface is provided in the cartridge case so that when the leader pin is inserted into the cartridge case, displacement of the pin retaining member due to the engagement of the pin retaining member with the leader pin is regulated within a predetermined range.