An apparatus for treating organic sludge by aerobic digestion has an aerobic digestion tank 1, a circulation line 6 provided with a pump for pumping circulated sludge, a liquid/gas ejector 7 for aspirating an oxygen-containing gas, which has been humidified, and for mixing it with the circulated sludge, a liquid/liquid ejector 8 for mixing the circulated sludge with sludge in a later step, a downward directing pipe 9 for passing downwards therethrough a gas/liquid mixture and an ozone-treated unit 31. The circulated sludge is mixed with an oxygen-containing gas in the liquid/gas ejector by jetting the circulated sludge into a throat 11 and passing the resulting mixture through the liquid/liquid ejector 8 and the downward directing pipe 9 to effect dissolution of oxygen into the sludge to cause aerobic digestion, while a gas/liquid mixed fluid composed of spent ozone gas and ozone-treated sludge is guided to an aspiration chamber 13 via a line 34 as humidified oxygen-containing gas in order to prevent clogging of the flow path in the aspiration chamber 13 due to drying of the sludge into solid crust.