The invention concerns a navigation device for a mobile robot comprising means for measuring the amplitude and the phase of an electromagnetic signal emitted by a wire acting as limit for a working area of the robot. The measuring means samples the amplitude of the signal during each time interval, the result of each measurement is stored in a memory and the measurements are repeated for several time intervals, the collected results being added in said memories until the content of a memory reaches a reference threshold. The number of samples required and the content of each memory is interpreted by numerical analysis to determine the distance or distance variation relative to said limiting elements. Any phase change corresponding to a passage beyond the limiting wire is easily detected and results for example in a command returning the robot to its working area.