PURPOSE: To improve the strength of foundation by bow-shapedly swelling the partial area of a circle drawn by an excavating blade from each side of the rectangle of a box-shaped casing, and mutually lapping the above bow- shaped parts and unifying them in the case of the adjacent execution of an improving pillar.

CONSTITUTION: The long side L2 of a box-shaped casing 300 is decided to be shorter than the twice length of its short side L1. The diameters D of excavating blades 110, 210 and of stirring blades 120, 220 are longer than the short side L1, and shorter than the twofold of a distance between the axial centers 0, 0 of shaft bodies 140, 240 and the corner of a rectangle. In the case where sediment is prepared by the casing 300 in improving the ground, a bow-shaped part is swelled from each side of a rectangular improving pillar. In addition, in the case of the adjacent execution of the rectangular improving pillar, the bow-shaped parts are mutually overlapped and unified. Thus the ground for structure foundation can be perfectly and wholly improved, and easily lappedly executed, and the strength of the improved foundation and resistance against earthquake horizontal force can be enlarged.