The distal end of each of a plurality of gas supply pipes 57 (only one of them is shown in FIG. 6) penetrates the hot-plate support member 5 to open into the gas-flowing space 5A. The proximal end of each gas supply pipe 57 is connected to a gas supply source 57 a, which stores a clean purging gas, such as an inert gas (e.g., nitrogen gas), to be used for cooling the hot plate 53.