Provided is a magnetic tape cartridge of a single reel type in which a magnetic tape is wound around a reel, in which the magnetic tape includes a non-magnetic support, and a magnetic layer including a ferromagnetic powder and a binding agent on the non-magnetic support, a tape thickness is equal to or smaller than 5.2 μm, a tape width difference (B−A) between a tape width A at a position of 10 m±1 m from a tape outer end and a tape width B at a position of 50 m±1 m from a tape inner end is 2.4 μm to 12.0 μm, and the tape width A and the tape width B are values measured 100 days from the date of magnetic tape cartridge manufacture.