Abstract: Soft soil stratum is stabilized by jetting powdery agent into the ground and mixing and agitating the same with the soil, wherein a rotary shaft is inserted into the ground and a powdery stabilized agent carried by air is pneumatically transported by way of a transportation tube positioned within the rotary shaft and is jetted out from a nozzle disposed in communication with a blade which is integrally attached to and extended from the bottom end of the shaft, is mixed and is agitated for solidification with the soil by the rotation of the mixing blade, while the carrier air jetted into the ground is induced and discharged to the outside of the ground after separation and filtration. The state of the supply of the powdery stabilized agent is continuously optimally controlled in accordance with a predetermined set condition. The powdery stabilized agent can be transported smoothly and mixed with the soil uniformly to provide satisfactory soil stabilization, without repair and maintenance problems.