A processing gas is prevented from entering into a space below a placement table. A supporting surface for supporting the lower face of a placement table is provided at an inner circumferential portion of the upper end of a support column. A circumferentially extending purge gas groove is formed outside the supporting surface, in an intermediate circumferential portion of the upper end of the support column. A narrow flow path is provided outside the purge gas groove, at a position corresponding to an outer circumferential portion of the upper end of the support column. A purge gas diffuses in the circumferential direction in the purge gas groove and flows out to the outside from the narrow flow path. Such a flow of the purge gas prevents a processing gas from entering into the purge gas groove and a space below the placement table.