PURPOSE: To suppress production of dust in a reaction chamber and improve the yield of a semiconductor device by filling the reaction chamber with inert gas, keeping the reaction chamber at specified pressure, and at the same time exhausting the air from the reaction chamber when a semiconductor substrate is transferred between the reaction chamber and an exhaust chamber.

CONSTITUTION: After etching, a reaction chamber 1 and an unloading chamber 6 are filled with inert gas such as N2 gas to set the pressure in them to 100 Pa. A semiconductor substrate 4 is transferred to the unloading chamber 6 in this state and at the same time the air is exhausted from the reaction chamber 1. Because the pressure in the reaction chamber 1 is high, the air is exhausted at a high speed. Therefore, a reaction product stays in the reaction chamber 1 for a short time and is efficiently discharged from the reaction chamber 1 without sticking to its inner wall. Thereby, since dust in the reaction chamber 1 decreases, excellent etching is performed and the yield of a semiconductor device improves.