A method for regulating an air-foam stabilizer for ground drilling by adding air foams and water or cement milk to soil having coarse and fine grains under drilling and mixing them, wherein the air foam amount and the water or the cement milk amount are controlled for stabilization on the basis of the following indices: <A> Defoaming water content ratio (Wmin) being the minimum water content at which when air foams are added to soil under drilling, defoaming would not occur, <B> Separation water content ratio (Wsep) being the maximum water content ratio at which any separation would not occur in the air-foam stabilizer, <C> Minimum air foam addition ratio (Qmin) exhibiting the minimum air foam addition amount from the viewpoint of the fluidity required for drilling, and <D> Maximum air foam addition ratio (Qmax) exhibiting the maximum air foam addition amount for obtaining the necessary density of the air-foam stabilizer as the minimum density required for drilling, and wherein, the defoaming water content ratio Wmin is a percentage computed by dividing the sum of the water amount corresponding to the surface dry moisture state of the coarse grains contained in the soil under drilling, and the water amount corresponding to the shrinkage limit of the fine grains, by the dry weight of the coarse and fine solid grains, and is expressed as the linear function of the fine grain content ratio, the separation water content ratio Wsep is a percentage computed by dividing the sum of the water amount corresponding to the separation water content ratio of the coarse grains contained in the soil under drilling, and the water amount corresponding to the separation water content ratio of the fine grains, by the dry weight of the coarse and fine solid grains, the separation water content ratio of the coarse grains is expressed as the linear function of the specific surface area of the coarse grains and the air foam addition ratio of the air-foam stabilizer, and the separation water content ratio of the fine grains is expressed as the liquid limit, the table flow value, indicating the fluidity relating to the drilling performance of the air-foam stabiliser, is controlled as the function of the water content ratio W in the air-foam stabilizer, the air foam addition ratio Q, the specific surface area S of the coarse grains of the soil under drilling, the fine grain content ratio P in the soil under drilling, and the liquid limit W L of the fine grains of the soil under drilling, in every four regions where the fine grain content ratio P of the soil under drilling is at least 10%, or less than 10%, and the air foam addition ratio thereof is at least 1 %, or less than 1 %, and the minimum air foam addition ratio (Qmin) is determined as the necessary air foam amount for maintaining the table flow value to be not lower than the control level when the water content ratio W in the air-foam stabilizer is the defoaming water content ratio (Wmin). <A> Defoaming water content ratio (Wmin) being the minimum water content at which when air foams are added to soil under drilling, defoaming would not occur, <B> Separation water content ratio (Wsep) being the maximum water content ratio at which any separation would not occur in the air-foam stabilizer, <C> Minimum air foam addition ratio (Qmin) exhibiting the minimum air foam addition amount from the viewpoint of the fluidity required for drilling, and <D> Maximum air foam addition ratio (Qmax) exhibiting the maximum air foam addition amount for obtaining the necessary density of the air-foam stabilizer as the minimum density required for drilling,