A disc cartridge comprising: approximately rectangular upper and lower shells (11, 12) making up a cartridge body by being separably combined; wherein with one of the upper and lower shells (11, 12), there are provided a first rectangular plate making up one principal face of the cartridge body, and a pair of left and right disc holders (41, 42) making up the side wall of the cartridge body provided to first side edges (40a, 40b) which face each other of the first rectangular plate, and second side edges (40c, 40d) which face each other, orthogonal to the first side edges (40a, 40b) to which the disc holders (41, 42) of the first rectangular plate are provided, are opened; and wherein with the other of the upper and lower shells (11, 12), there are provided a second rectangular plate making up the other principal face of the cartridge body, a pair of back and forth standing wall members making up the front face and the rear face of the cartridge body, which are provided to third side edges which face each other of the second rectangular plate, and a dividing wall making up a portion of the side face of the cartridge body, which is provided to fourth side edges orthogonal to the third side edges provided to the standing wall members of the second rectangular plate; and wherein each of the pair of disc holders (41, 42) is in parallel with the first rectangular plate, at least one supporting groove slidably supporting the outer circumferential edge of a disc in the face direction of the disc is formed across from one edge to the other edge, a disc insertion/ejection opening where the disc is inserted/ejected from one of the second side edges is provided to one edge, and a disc push-out opening into which a push-out member of a disc device for pushing out the disc from the other of the second side edges (40c, 40d) to the disc insertion/ejection opening side enters is provided to the other edge; and the second side edges (40c, 40d) are closed with the pair of the standing wall members (10a,10b) by the upper and lower shells (11, 12) being combined together; and the second side edges are opened, and the disc insertion/ejection opening and the disc push-out opening are opened, by the upper and lower shells (11, 12) being separated in the vertical direction orthogonal to each principal face of the first rectangular plate and the second rectangular plate, wherein a releasable locking mechanism combines the upper and lower shells (11, 12) and is located within the cartridge outside the plane of projection of the disc. approximately rectangular upper and lower shells (11, 12) making up a cartridge body by being separably combined; wherein with one of the upper and lower shells (11, 12), there are provided a first rectangular plate making up one principal face of the cartridge body, and a pair of left and right disc holders (41, 42) making up the side wall of the cartridge body provided to first side edges (40a, 40b) which face each other of the first rectangular plate, and second side edges (40c, 40d) which face each other, orthogonal to the first side edges (40a, 40b) to which the disc holders (41, 42) of the first rectangular plate are provided, are opened; and wherein with the other of the upper and lower shells (11, 12), there are provided a second rectangular plate making up the other principal face of the cartridge body, a pair of back and forth standing wall members making up the front face and the rear face of the cartridge body, which are provided to third side edges which face each other of the second rectangular plate, and a dividing wall making up a portion of the side face of the cartridge body, which is provided to fourth side edges orthogonal to the third side edges provided to the standing wall members of the second rectangular plate; and wherein each of the pair of disc holders (41, 42) is in parallel with the first rectangular plate, at least one supporting groove slidably supporting the outer circumferential edge of a disc in the face direction of the disc is formed across from one edge to the other edge, a disc insertion/ejection opening where the disc is inserted/ejected from one of the second side edges is provided to one edge, and a disc push-out opening into which a push-out member of a disc device for pushing out the disc from the other of the second side edges (40c, 40d) to the disc insertion/ejection opening side enters is provided to the other edge; and the second side edges (40c, 40d) are closed with the pair of the standing wall members (10a,10b) by the upper and lower shells (11, 12) being combined together; and the second side edges are opened, and the disc insertion/ejection opening and the disc push-out opening are opened, by the upper and lower shells (11, 12) being separated in the vertical direction orthogonal to each principal face of the first rectangular plate and the second rectangular plate, wherein a releasable locking mechanism combines the upper and lower shells (11, 12) and is located within the cartridge outside the plane of projection of the disc.