A recording disk cartridge includes a plurality of flexible recording disk media integrally rotatably housed within a cartridge case including: a lower plate for configuring a lower wall; at least one inner plate partitioning the recording disk media; and an upper plate for configuring an upper wall, wherein the cartridge further comprises a center core provided for each recording disk medium, having a main body portion and a collar part widening in an outer diameter direction from the main body portion; each recording disk medium is affixed to the collar part of the respective center core; and each inner plate has a central opening configured to accommodate the main body portion of the center core and has a recess part along a lower rim of the central opening for preventing interference with the collar part of the center core disposed on a layer immediately below.