PURPOSE: To avoid creeping of a purge gas to a surface to be treated of a workpiece by forming a rear surface of an inner peripheral edge of a ring covering a peripheral edge of the surface to be treated of the workpiece higher than a press surface for pressing the workpiece by forming a small gap between the peripheral edge and the surface to be treated of the workpiece.

CONSTITUTION: When a peripheral edge of a surface of a wafer is covered with a press ring 51, its outer peripheral edge protrudes to an outward side of the wafer. A superposed part A of the ring with the wafer is formed to reach about 4-6mm and a protruding part B is formed to reach about 20-30mm so as to cover the film forming part of the wafer with the inner peripheral edge. A contact part 52 is formed at its lower surface as a press surface 53 for pressing the peripheral edge of the wafer. A uniform gap is formed between a rear surface of the ring 51 and the surface of the wafer.