The inventioin discloses a recording tape cartridge capable of keeping position error signals during running of a recording tape small. The recording tape cartridge is provided with a reel hub, a magnetic tape that is wound round the reel hub, and a lower flange and upper flange that are disposed to oppose one another at each of the both axial direction ends of the reel hub. In a state in which the magnetic tape is completely wound on the reel hub, the magnetic tape is offset toward the upper flange. A space of an edge clearance between a lower end of an outermost periphery portion of the magnetic tape and the lower flange is at least 0.18 mm and at most 0.46 mm. A rate of widening of a facing distance between the lower flange and the upper flange increases toward the outer periphery side.