To provide a locking/unlocking device of a plug or a lid on a frame using a key, having less number of components than the components used in existing devices.

The locking/unlocking device of a plug or a lid on a frame using a key, in which a component as a latch 9 projects from the plug 1, has an elastically deformable rod 12 with a hook-shaped free end 12a which fits over a hard rib 2a of a receiving surface of the frame 2, and includes furthermore a lug 8 in a spur shape enabling to resiliently let the rod 12 detach so that a key 3 releases the hook-shaped free end 12a from the hard rib 2a.