PURPOSE: To accelerate the deposition rate by a method wherein a mixing chamber is provided in a pressure vessel to mix one or more kinds of material gasses while an intermediate product produced by vapor reaction is exhausted from a gas exhaust port.

CONSTITUTION: Wafers are processed by leading a material gas from tubular bodies 5, 6 into a mixing vessel 24. At this time, the pressure inside the vessel 24 is boosted higher than that inside a reaction vessel 9 by the conductance of a gas exhaust port 25. Accordingly, two kinds of material gasses are vapor- reacted in the mixing vessel 24 at higher pressure than that in the vessel 9 so as to accelerate the production of intermediate product in high concentration. Therefore, the intermediate product is led from the exhaust port 25 into the reaction vessel 9, thus enabling the normal films to be formed in the pressure reduced state. Through these procedures, the deposition rate can be accelerated simultaneously forming excellent films and removing the cause of raising dust.