A robot capable of teaching operation is installed in the vicinity of a transfer line in which a plurality of vehicles are successively transferred. The robot is provided with a roller which is rotatable and made of a material capable of absorbing and storing a protective layer forming material. A controller for controlling the robot receives a production control information from a production control information supply section and recognizes the number of coating-unnecessary vehicles continuously transferred, on the basis of production control information (step S201). When the number of coating-unnecessary vehicles successively transferred is three or more, the roller is moistened (step S203). When the number of coating-unnecessary vehicles successively transferred becomes two, moistening is terminated and a preparatory operation is performed (step S207).