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.