We have developed a sensor systemthat can simultaneously detect the travel position and attitude of amobile robot. The developed sensor system consists of magnet sensors and N landmarks set up on the road. The N landmarks, made of steel plate, are set up in the shape of an N on the road. The magnet sensors accurately detect the center of a steel plate and calculate the center of the N landmark and the attitude of the mobile robot. The sensor system realizes detection that is robust against lift off variation and deposition (soil, water, snow, etc.) on the steel plate. We have confirmed the validity of the sensor system through a travel experiment involving a mobile robot on which the sensor system is mounted.