A cartridge comprises a case consisting of upper and lower shells joined and containing a medium, a shutter which slides astride over one edge portion of the case to open or close a slot formed in the case to expose the medium partly, and a locking member which keeps the shutter in the closing position immovably. The locking member is normally, except when unlocked, exposed partly in a recess for the access of an unlocking member, so that a push of the exposed portion of the locking member disengages it from a projection formed on the inner side of the outer edge of the shutter. Confining structure is provided for confining the position of the exposed portion of the locking member always to the inside of the line extended from the end face of the shutter is provided between the exposed portion and the interior surface of the case.