Abstract: The present invention provides a closure mechanism applied to a cartridge inserting opening in a floppy disk drive or the like and including a lid member rotatably supported on a panel having an opening through which a cartridge or the like can be inserted into the floppy disk drive, the lid member biased toward its closed position under the action of a biasing spring which is rigidly mounted at one end on the panel with the opposite free end being engaged by the lid member under a biasing pressure such that the biasing force on the lid member can be maintained substantially constant by slidably moving the free end of the biasing spring on the surface of the lid member when the lid member is being opened.