PROBLEM TO BE SOLVED: To efficiently form a ground improvement pile with a diameter large enough to support a building.SOLUTION: A method for constructing a ground improvement pile comprises the steps of: excavating a foundation G downward to a predetermined depth by a rotary excavation jig 5 with a first diameter; excavating the foundation G upward by a rotary excavation jig 7 with a second diameter larger than the first diameter; and packing a filler in an excavation hole H excavated in the respective steps and mixing the excavated sediment generated by excavation with the filler and hardening the mixture so as to form an improvement pile with a predetermined depth. In the method, in the case where an adjacent excavation hole H is already formed, at the time of excavating by the rotary excavation jig 5 with the first diameter, the outer peripheral edge of the rotary excavation jig 5 with the first diameter and the adjacent excavation hole H are separated.SELECTED DRAWING: Figure 1
