A processing gas is prevented from entering into a space below a placement table. A supporting surface 62 for supporting the lower face of a placement table 58 is provided at an inner circumferential portion of the upper end of a support 56. A circumferentially extending purge gas groove 64 is formed outside the supporting surface 62, in an intermediate circumferential portion of the upper end of the support 56. A narrow flow path 68 is provided outside the purge gas groove 64, at a position corresponding to an outer circumferential portion of the upper end of the support 56. A purge gas fed from purge gas-feeding means 66 into the purge gas groove diffuses in the circumferential direction in the purge gas groove 64 and flows out to the outside from the narrow flow path 68. Such a flow of the purge gas prevents a processing gas from entering into the purge gas groove 64 and a space S1 below the placement table.