PURPOSE: To use a solar collector to its full capacity irrespectively of summer and winter and improve fermentation efficiency by raising the temp. in a fermentation tank to middle temps. in a winter time and to high temps. in a summer time to perform high temp. fermentation by the solar heat collector.

CONSTITUTION: The methane fermenting device is provided with a solar heat collector 8 of which the quantity of heat collection in a winter time is equal to the quantity of heat necessary for raising the temp. in a fermentation tank 1 to middle temps. (33W38°C). Hence, in a winter time, the temp. in the tank 1 rises to middle temps. and in a summer time, it rises to high temps. (50W55°C). In a winter time, middle temp. fermentation is caused by using middle temp. germs, while in a summer time, high temp. fermentation is caused by using high temp. germs. This fermentation method is particularly suitable for formation of gaseous methane by anaerobic fermentation of org. waste materials such as garbage and poultry waste. Here, 2 in the figure denotes a heat exchanger, 3 a temperature sensor and 5 a hot water storage tank, respectively.