Disclosed herein is a disk cartridge including a disk; a cartridge body in which the disk is rotatably accommodated, the cartridge body having a recording and/or reproducing window for exposing a part of the disk; a shutter mounted on the cartridge body so as to be movable between a closed position where the window is closed by the shutter and an open position where the window is opened by sliding the shutter; a shutter locking member adapted to engage a locking portion of the cartridge body when the shutter is in the closed position, thereby limiting the movement of the shutter in the direction of opening the window; a biasing member provided on the shutter for biasing the shutter locking member in the direction of engaging the shutter locking member with the locking portion of the cartridge body; and a deformation preventing member provided in the cartridge body at a position inside of the biasing member for preventing the deformation of the biasing member inward of the cartridge body.