PURPOSE: To keep always a substrate at a suitable temperature without being affected by change in substrate input heat during treatment in the plasma process of a semiconductor substrate, and stably realize treatment like formation of a high fidelity pattern of a mask or high quality film.
CONSTITUTION: A substrate mounting electrode 3 is equipped with a thermometer 11 capable of measuring in-situ the temperature of a substrate 1 during treatment. An operation circuit 18 and a heat conduction gas supply pressure control system 12 are installed. The operation circuit 18 calculates the following in the real time on the basis of the temperature signal from the thermometer 11; the reached equibrium temperature for the substrate and the heat transmittance between the substrate and the substrate mounting electrode during treatment. The control system 12 adjusts the pressure of heat conduction gas spplied to a gap between the substrate and the substrate mounting electrode, based on the operation results of the circuit 18.