PURPOSE: To miniaturize and integrate the whole of equipment and reduce the equipment expense to a large extent by installing a methane fermentation tank for receiving oxidated fermentation liquid which has completed the processes of oxidative fermentation and the like and carrying out the methane fermentation on the outer periphery of an oxidative fermentation tank and constituting a fermentation device main body of double structure.

CONSTITUTION: A fermentation device main body 50 of double structure is composed by installing a methane fermentation tank 14 for receiving oxidatively fermented liquid which has completed the processes of oxidative fermentation and performing the methane fermentation on the outer periphery of an oxidative fermentation tank 10 for dissolving an organic matter by means of hydrolysis, oxidative fermentation or the like. The oxidative fermentation tank 10 and the methane fermentation tank 14 are separated into two tanks, because it is better to perform the methane fermentation more effectively and stably after removing carbon dioxide gas preliminarily when the action of anaerobic methane composing fungi is disturbed by the carbon dioxide gas. A large amount of organic matter raw material can be treated in a methane fermentation tank 14 of a small volume by separating the device into two tanks.