A magnetic tape cartridge includes an opening 1C through which a magnetic tape is drawn out, and a slide door 6 that slides along a sidewall 1D provided with the opening 1C to open and close the opening 1C. The opening 1C is provided in part of the sidewall 1D across a lower half 1A and an upper half 1B of the cartridge case 1. At least one of a lower half portion and an upper half portion of the sidewall is provided with a holding groove 1L that receives a front end portion 6A of the slide door 6 when the opening is closed. Therefore, the upper half 1B and the lower half 1A of the magnetic tape cartridge may be smoothly combined.