Before supplying ammonia, which is a flammable gas, into a chamber, the chamber is evacuated to eliminate residual gas from the chamber, and then nitrogen is fed into a combustion-supporting gas line to cause oxygen, which is a combustion-supporting gas, remaining in the combustion-supporting gas line to flow out. After supplying ammonia into the chamber, the chamber is evacuated to eliminate the ammonia from the chamber, and then nitrogen is fed into a flammable gas line to cause the ammonia remaining in the flammable gas line to flow out. Oxygen or ammonia and nitrogen are alternately eliminated on the downstream side of an exhaust part, and it is therefore possible to prevent the flammable gas and the combustion-supporting gas from being mixed with each other.