Abstract: The invention relates to a device for inspecting at a distance walls which are difficult of access such as metal walls forming a nuclear reactor vessel. The device comprises a detector secured to a slide movable along guide rails attached to a frame. Probes bearing on the metal walls are secured to a plate on the frame, each probe transmitting a signal proportional to the distance between the guide rails and the place where the probe bears on the part of the wall to be checked. The device includes two motors secured to a second frame and which are controlled by electrical signals from the probes. Each motor actuates a pinion engaging the racks secured by pivots to the first frame and the two ends of the guide rails. A potentiometric monitoring device stops the motors when the signals proportional to the distance between the wall and guide rails as measured by the probes are equal to one another and to a controlled value. The detector preferably comprises an assembly of transducers and ultrasonic receivers.