PURPOSE: To facilitate an effective and speedy methane generation, by carrying to a fermentation vessel an effluent generated by the decomposition of solid organics in a decomposition vessel, and heating and digesting the effluent in the fermentation vessel.

CONSTITUTION: Solid organics 16 of big sizes, e.g. garbages from kitchens, are charged into a decomposition vessel 1, where the organics 16 are subjected to methane fermentation. An effluent 17 generated by the methane fermentation in the vessel 1 is carried through a conduit 3 to a fermentation vessel 2. In the vessel 2, the effluent is digested to generate methane gas, while being heated with a heater 13. The gases generated in the vessels 1 and 2 are taken out from a valve 5 through a gas conduit 4. Consequently, methane generation is facilitated with saved energy, because only a pertinent part is heated.