Abstract: A baking oven is divided into a first baking oven and a second baking oven. A conveyor line is also divided into two sections, a first conveyor line section being so disposed as to pass through the first baking oven only, and a second conveyor line section being so disposed as to pass through the second baking oven only. A substrate is coated with a paint and then conveyed through the first baking oven while being so supported by a carrier with a rotating device and being rotated about its approximately horizontal axis extending in a longitudinal direction thereof with the rotating device. The rotation of the substrate is carried out at a speed which is high enough to prevent the paint coated from sagging due to gravity yet which is low enough so as to cause no sagging as a result of centrifugal force.