PURPOSE: To restrain pressure fluctuation in a process chamber at the time of switching gas feeding lines.

CONSTITUTION: In a method for supplying gas to a chamber 3 wherein reaction gas and inert gas are alternately supplied to the chamber 3 from a reaction gas feeding line 1 and from an inert gas feeding line 2, respectively, branching valves V3 and V4 are connected with the primary sides of switching valves V1 and V2 installed in the respective lines 1 and 2, respectively. A vent line 6 is connected with the branching valves V3, V4. The switching valve V1 of the reaction gas feeding line l and the branching valve V1 of the inert gas feeding line 2 side, and the switching valve V2 of the inert gas feeding line 2 and the branching valve V3 of the reaction gas feeding line 1 side are simultaneously opened and shut, respectively. While the vent line 6 is vacuumized, reaction gas and inert gas are alternately supplied to the chamber 3 from the reaction gas feeding line 1 and the inert gas feeding line 2.