A disk cartridge is provided which includes a cartridge body (2) including a lower shell (6) having formed therein a first opening (24) for write and/read and an upper shell (7) butt-joined to the lower shell (6), a rotation wheel (4) housed rotatably in the cartridge body (2) and which houses a disk (3) and has formed therein a second opening (44) corresponding to the fist opening (24) and through which the housed disk (3) is exposed to outside, a pair of shutter plates (5a, 5b) provided between the lower shell (7) and rotation wheel (4) and which operates with the rotation of the rotation wheel (4) to uncover the first and second openings (24, 44) when the latter are coincide with each other, and a locking mechanism (71) including a locking portion (76) installed pivotably to the upper shell (6) and engaged in an engagement concavity (85) in the rotation wheel (4) to lock the rotation wheel (4) in a position where the pair of shutter plates (5a, 5b) covers the first opening (14), and an unlocking portion (77) which cancels the engagement of the locking portion (76) in the engagement concavity (85).