The ceiling dismantling robot that we developed cuts ceiling boards with a high-pressure water-jet cutter. The cutting operation of the ceiling board is done below the Light-Gauge Steel (LGS) attached to ceiling boards with screws. A magnetic sensor that detects LGS was developed for this robot system. The robot can dismantle ceiling boards because of a successful mock-up experiment. This paper describes the method of screw detection that targets saving water and reducing dust. There are several problems, for example, increased noise and detecting objects as early as possible. The validity of the proposed method was verified by detecting the screws using sensor in an experimental setup.