To provide a recording medium driving device for preventing a recording medium inserted in an abnormal direction from being further erroneously inserted at an early stage.

When a medium case 1 is inserted in a normal direction, the medium case 1 can be smoothly inserted because protrusions 3a and 3c, and protrusions 3b and 3d pass through on an inner side between the base face 21D and the base face 21E of a chassis toward a moving direction and are not abutted against the base face 21D and the base face 21E. On the other hand, when the medium case 1 is reversely inserted with the front and the back of it exchanged, the medium case 1 can be prevented from being further erroneously inserted at an early stage because the one protrusion 3c formed on the bottom face of the medium case 1 is abutted against the base face 21E formed at the insertion/ejection opening side of the chassis 21.