Measuring relative positions among robots are essential to avoid collision and to coordinate one another. This paper proposes a signboard system and an algorithm to measure the relative positions among mobile robots in realtime. The system consists of a CCD camera and signboards with LEDs, whose 2D images are obtained by a CCD camera loaded on a different robot. The algorithm first separates the image into each signboard, and second detects the position and the orientation of the target robot. Error analysis made it possible to measure the orientation of the target robot in 360 degree with the minimum errors. The experiments with 3 mobile robots indicates that one robot can measure the other two position with the accuracy of 5% in distance within 0.4 second, in the range of 0.5 to 3 meters. The measuring speed is fast enough to calcuate the collision avoidance. The experiments verified the system is very efficient.