To provide a small magnetic disk cartridge of extra-thin type and having a large recording capacity, which is prevented from horizontal slippage when being piled.

The cartridge cases 1A and 1B consists of a base plate and a pair of cover cases 3 and 4 adhered to the upper/lower surfaces of the base plate in the mode of exposing the four corner parts 27 of this base plate. One 27b of the upper/lower surfaces of the four corner parts 27 is made a projecting part and a recess part to be engaged with the projecting part of the cartridge adjacent when the cartridges are piled plurally is formed at the other part 27a.