To endow such a device, of the type with a bolt (1) actuated by a key (2), with a high level of tamper-proofing and security without compromising its simplicity and reliability, the device includes two bolt-guiding members (3, 4) carried by one of the two elements, and the bolt includes a key-inserting aperture (114), two guiding ramps (112, 113) cooperating with the guiding members, and a third guiding ramp (115) cooperating with the key (2) when the latter is inserted into the aperture (114), in order to selectively actuate the bolt (1) from a locking position to an unlocking position, and vice versa, by rotation of the key. Use: devices for closing roadway or footway manholes, or underground technical chambers.