To improve consumption at a value suitable for a chemical fertilizer and to facilitate recycling by vigorously and aerobically degrading hardly decomposable organic wastes considered to be hardly microbially decomposed stably for a long period, further efficiently performing ripening degradation until the volume is reduced down to 1/50 of the charge, and enhancing the dry degradation rate of the organic matter.

The hardly decomposable organic wastes restricted to about 7 kg (6 to 10 kg) per m3 are charged to a deposition layer of a prescribed height in a hermetic vessel atop a wide area of a main fermentation chamber occupying about 75% (60 to 85%) of the deposition layer and are evenly agitated in the main fermentation chamber. The agitation is electrically controlled so as to thinly mix the wastes. In a ripening chamber for the remaining volume, the wastes are uniformly agitated, are successively transferred and are made to flow over to a storage chamber. Uniform ventilation from the under surface of the deposition layer to the main fermentation chamber and the ripening chamber is surely performed.