To reduce deposited clay and generated surplus soil and enhance agitation effect and prevent the collapse of a hole wall of negative pressure and construct an underground wall with high accuracy which is free from bending or twisting.

A screw blade 3 forming a through hole 10 and a plate-like agitation blade 4 are properly laid out on each excavation rod 2 of a multi-shaft kneader auger machine 1. The screw blade 3 is provided with the large through hole 10 in the central part and fan-shaped with a ring-shaped peripheral edge left over so that they may be lined up and mounted. The agitation blades 4 are lined up as a pair alternately in every direction from the excavation rods 2 so that they may be projected on all sides as viewed on the plane. The screw blades 3 are turned by turning the excavation rods 2 so that mud may pass through the through hole 10, thereby minimizing the generation of surplus soil without pushing up the mud. Even large clods of earth are squeezed so that they may be well agitated. In addition, only a small amount of clay is deposited on the screw blade 3. When the excavation rod 2 is pulled out, the mud passes through the through hole 10, thereby preventing the collapse of the hole wall attributable to the negative pressure and reducing cleaning work as well.