PURPOSE: To automatically polish a curved surface by detecting the slope of the surface of an article to be polished, by a sensor group and raising and lowering a sander with a truck by the operation of a hydraulic cylinder and allowing the sander to follow the surface of the article to be polished and freely tilting the truck along the surface of the article to be polished.

CONSTITUTION: A truck 9 is connected to the top edge of the piston rod 7 of a fluid cylinder 5 through a universal joint 8, and a sander 10 is installed onto the truck 9. Among three sensors 15, 16, and 17 for the truck 9, the sensor 16 is arranged higher by 1mm than the sensors 15 and 17. In polishing for a flat surface, the sensors 15 and 17 contact with the surface of an article 1 to be polished, and the sensor 16 does not contact with the article 1 to be polished. When all the sensors 15W17 contact with the article 1 to be polished on a plane having a slope, a fluid cylinder 5 operates to pull-up or lower the piston rod 7, and the truck 9 is vertically moved or tilted according to the slope, and the sander 10 always follows to the surface of the aricle 1 to be polished.