PURPOSE: To facilitate the maintenance for gas leakage and remarkably increase safety, etc., by providing a sucking device in a gas discharge side pipe line and feeding and discharging raw gas or medium gas while a pressure in a reaction vessel and pipe lines is made negative successively when a gas reaction device is operated.

CONSTITUTION: A reaction vessel 1 in which a closing cover 2 is provided through a seal 3 is located between a gas inlet side pipe line 4a and a gas discharge side pipe line 4b. The gas inlet side pipe line 4a is provided with a gas inlet section 9a for raw gas and a medium gas generating section 8, and the gas discharge side pipe line 4b is provided with a sucking device 5a, a gas processing section 5b, and a gas discharging section 9b. Also, a valve mechanism 5c such as a check valve, etc., for regulating gas flow in a specified direction is provided in the pipe lines 4a and 4b. Next a pressure in the reaction vessel 1 is reduced by the sucking device 5a in the gas discharge side pipe line 4b, and raw gas is introduced from the gas inlet section 9a in the gas inlet side pipe line 4a. Then medium gas is generated in the medium gas generating section 8, and filled into the reaction vessel 1.