<p>A disk tray (5) of a disk cartridge (1) has a bottom plate (31) having formed in it a tray opening (35) enabling disk access; a ring-like sidewall (32) raised at an outer edge of the bottom plate (31); a circular arc-like opening connection section (36) formed so as to be continuous from the side wall (32) and straddling across the tray opening (35) in the bottom plate (31) to reinforce the opening; and a projection (39) projecting radially outward of the disk tray (5), along the side wall (32), from one end in the circumferential direction of the opening connection section (36), and constructing a part of a recess (37) engaging with a drive side member when the disk tray (5) is turned. The projection (39) is of a hollow structure or a lightened structure, and a gap (S) is formed between an end on at least the opening connection (36) side of the projection (39) and an outer peripheral surface of the opening connection section (36).</p>
