PURPOSE: To make it easy to remove crystals formed on an outside surface of a carbon plate in the most outward position from heat treatment gas by mounting a quartz plate on the carbon plate in the most outward position.

CONSTITUTION: Quartz plates 16, 17 to be mounted by mounting screws 16a, 17a are provided on an outside surface of carbon plates 11', 11'' in the most outward position among a plurality of carbon plates 11. After a semiconductor wafer M is provided on a wafer boat 10, an appropriate high-frequency current is supplied from an outside high-frequency power supply to electrode plates 14, 15. This initiates discharge between adjacent carbon plates 11, and a chemical deposition film is formed on the surface of the semiconductor wafer M. After the process is terminated, the quartz plates 16, 17 are removed from the carbon plates 11', 11'' in the most outward position of the wafer boat 10 and this is washed by a mixed acid of hydrofluoric and nitric acids. As a result, crystals formed on the surface in the step of forming chemical deposition films are removed.