A recording disk cartridge includes a plurality of flexible recording disk media integrally rotatably housed within a cartridge case. Each of the recording disk media is fixed to a center core which is stacked one on another through a spacer. The cartridge case includes: a lower plate for configuring a lower wall parallel to the plurality of the recording disk media; at least one inner plate that is stacked and fixed on the lower plate, and partitions the plurality of the recording disk media; and an upper plate that is stacked and fixed on the inner plate, and configures an upper wall of the cartridge case. Each spacer is provided with locking protrusions at upper and lower surfaces thereof, and each center core is provided with locking holes for engagement with the locking protrusions of adjacent spacers.