The dismantling robot with highly accurate positioning was developed and the robot must cut the plasterboard that is fixed to light gauge steel with the screw bolt under the plasterboard, so the sensor with magnets to detect and trace the light gauge steel was designed. The developed sensing method has detecting mode and tracing mode to keep high position accuracy and high scanning speed. It is installed to the dismantling robot and tested for detecting and tracing the light gauge steel. The test showed that the sensing system and design method is practically useful.