An inner member having a supply port is installed at the inside of a housing, wherein the housing and the inner member are connected together by connecting bolts. Air is supplied to the supply port of the inner member and is directed and introduced through communication passages into an annular passage. Thereafter, the air is directed outwardly from a plurality of lead-out holes, which communicate with the annular passage, in a swirling direction with respect to an annular recess facing the workpiece. In addition, by having the air flow along the annular recess, which is formed with a substantially trapezoidal shape in cross section, the workpiece is maintained in a non-contact state with respect to a holding surface of the inner member.