Abstract: A soil improvement apparatus where an auger head is rotated even with small power and where an improvement material is extruded only in the horizontal direction. The soil improvement apparatus has soil compacting means that is constructed by inserting an auger in a tubular casing. The auger has a rotating shaft and a rotating tube that are coaxially arranged and rotate independently in the circumferential direction, an auger head attached to the lower end of the rotating shaft, and a screw provided on the outer periphery of the rotating tube. The auger head is constructed by arranging wedge-shaped auger blades, having recessed pressing sections, on the lower end of the rotating shaft projecting from the casing, and the auger blades are arranged in a radial pattern with the pressing sections facing the radial direction.