The inventors of this invention conducted a test and found out that to prevent peel-off of an adherent film, it is not of essential importance to set the radius of curvature equal to or larger than a predetermined threshold. The inventors of the present invention also found out that peel-off of an adherent film occurs in the region in which the curvature of a shield changes and is less likely to occur when the change in curvature of the shield is small. Accordingly, the key to the problem is the magnitude of a change in curvature of the shield, so changing the curvature stepwise makes it possible to suppress a large change in curvature, and thus to prevent peel-off of an adherent film free from any disadvantages such as deterioration in film thickness distribution, which may occur due to an increase in size of the shield.