PURPOSE: To provide a single aerophobic tank in which both methane fermentation and deoxidation processes take place in a microbial phase.

CONSTITUTION: Gas collected in a gas holder 5 is circulated using an air pump 4. A deoxidation device is provided halfway through a circulation path and oxygen is removed from the gas by the device. The gas free from oxygen is introduced into a bacterial strain culture tank 1 and then is bubbled. A culture liquid nurtured in the bacterial culture tank 1 is added to an aerophobic tank 2 as a microbial strain. The addition is performed at the time of starting the air pump or intermittently after the startup.