Maintenance device for working in a maintenance region of a plant delimited by at least one wall (32) having at least two facing surfaces between which the maintenance work is performed and which is accessible from an introductory region (31) of the plant, comprising a robot (2) for carrying out the maintenance work in a remote-controlled or programmed way and an immobilizing device for immobilizing a supporting part (2a) of the robot (2) in the work position, comprising additionally a motorized and remote-controlled transport carriage (4) and wherein the device (3) for immobilizing the supporting part (2a) of the robot (2) is a clamping support secured to the transport carriage (4), comprising at least two clamping shoes (11, 13a, 13b) associated with movement means (12, 14a, 14b), each in at least one direction so as to move the shoe (11, 13a, 13b) away from the clamping support (3), the movements of the two clamping shoes (11, 13a, 13b) being performed in parallel and opposite directions, and removable means (9) of fixing the supporting part (2a) of the robot (2) on the motorized transport means and the robot (2) is a robot arm comprising elements (2b, 2c, 2d, 2e, 2f) mounted to rotate one with respect to another and produced in the form of an anthropomorphic arm.