PURPOSE: To prevent the contamination of the surface of a wafer, by providing a hole part facing toward the surface of a the wafer in a cap body attached to a central shaft, and blowing off an air stream on the surface of the wafer through the hole part.

CONSTITUTION: A hole is provided at the center of a rotary table 1. A wafer 2 is centered with a central shaft 4. A cap body 5 is attached on the central shaft 4. The cap body 5 comprises a cap-body upper part 6 and a cap-body lower part 7. High-pressure nitrogen gas is supplied through the hole beneath the central part of the table 1. Finally the nitrogen gas is blown off on the upper surface of the wafer 2 through a blowing port 8. Thus the contamination of the surface of the wafer 2 is prevented.