PROBLEM TO BE SOLVED: To produce mature soil in which fermentation treatment is evenly performed, and furthermore, the generated mature soil having a microorganism phase in which anaerobes and aerobes coexist.SOLUTION: An organic reproduction material in which organic waste and inoculum mature soil, whose microorganism phase is adjusted, are mixed is charged into a fermentation tank 1, air is circulated from a lower portion of the organic reproduction material through the organic reproduction material, and the organic waste in the organic reproduction material is fermented. Fermented steam generated in an upper portion of the organic reproduction material associated with fermentation is separated into air and liquid using a filter 9a, the separated air is returned into the fermentation tank, and circulated through the hermetic fermentation tank 1 through the organic reproduction material. The fermentation tank 1 has a plurality of air supply pipes 5 in parallel at a predetermined gap under the floor of the tank, each air supply pipe 5 has valves for air flow rate adjusting devices 5a to 5e, and the valves are adjustable using the air flow rate adjusting devices to bring flow rate of air flowing in each air supply pipe approximately to the same.
