To improve fermentation decomposing efficiency of a raw sludge by a method wherein a lowest limit temperature in a fermenter is kept at a specific temperature for a specific period of time after start of operation and thereafter, it is kept at another specific temperature.

A saw dust as a fungus-supporting medium is charged into a garbage treating machine with a heater having a stirring vane, and the garbage is treated by decomposition under a condition of about 40% in water content. At that time a lowest limit temperature in a fermenter is maintained at 30 to 40°C for 5 to 10 days from start of operation. For example, it is 40°C for 7 days. Thereafter, it is 50 to 60°C, for example 50°C. By this treatment, while the lowest limit temperature in the fermenter after start of operation is set at 30 to 40°C, thermophilic bacteria are gradually multiplied, activity of the thermophylic bacteria is increased by rise of the temperature to 50 to 60°C thereafter, and decomposition by fermentation can be efficiently executed.