on the outside of the back cover 26 on which the substrate 25 is arranged to forcedly cool the back cover 26 by circulating cooling water in the water cooled tube 30 another water cooled tube 28 is arranged on a lower surface of the base 6 for cooling the base 6 an effect of cooling the back cover 26 can also be achieved by providing a forced air cooling device such as a fan in place of the water cooled tube 30 in a connecting portion 27 a and a contacting portion 27 b between the back cover 26 and the base 6 a heat insulating material 27 with low heat conductivity is arranged the heat insulating material 27 which is an element for improving an effect of cooling the back cover 26 and an effect of heating the base 6 serves to interrupt the transfer of the heat of the base 6 heated by the heater 29 to the back cover 26 the heat insulating material is formed of a heat insulating ceramic material for example ko ntio cao or sio or resin for example fluorine contained resin acrylic resin epoxy resin or other high temperature resins or a metal with low heat conductivity for example stainless steel or chromium nickel alloy 18cr 2 ni the process gas sucked through the gas intake port 3 moves in the gas discharge path toward the gas discharge port 4 while the temperature thereof decreases however since the base 6 is heated by the heater 29 the process gas can be prevented from adhering and accumulating near the base 6 as a solid product also since the substrate 25 is.