Ground improving work for soft ground that does not involve discharge of sludge containing cement milk onto the ground or causing environmental problems is realized by rotating a soil excavating tool having a leading pipe, a shaft body and a spiral vane attached to the periphery of the shaft body, the portion of the shaft body where the spiral vane is attached being thick in the central region and tapered at the opposite ends along the axial direction, the upper and lower surfaces of the spiral vane being provided with pawls, excavating the soil while spouting compressed air from the leading pipe, reversely rotating the soil excavating tool when a predetermined depth is reached, injecting cement milk from the spiral vane while stirring the soil so as to mix the soil and cement milk, thereby forming an improved body in the soil.
