Abstract: A spin holder has a disk-like supporting portion, and a circular wall portion for supporting the back side of a substrate is formed inside the periphery of the top surface of the disk-like supporting portion. A circular groove is formed near the periphery of the top surface of the disk-like supporting portion, the circular groove surrounding the outside of the circular wall portion. An O ring is fitted into the circular groove. The top surface of the circular wall portion on the spin holder supports the back side of the substrate and the O ring comes in close contact with the back side of the substrate. Even if a vacuum leakage occurs between the circular wall portion and the back side of the substrate due to suction force created by a vacuum suction source, the O ring keeps the hermetic state in the space between the circular wall portion and the O ring. This prevents mist around the spin holder from being drawn toward the suction surface between the O ring and the substrate.