The placement of a wafer can be performed with good positional precision, and the positioning of wafer clamps during maintenance is facilitated. A tapered recess is formed in that portion of the wafer support on which a wafer is placed. When a wafer is placed in the recess, the sloped surface inside this recess comes into contact with the edge of the wafer from beneath the wafer. The wafer is supported in a specific attitude by the sloped surface inside this recess.