PURPOSE: To prevent the crumbling of the wall of a pit by a method in which the pit is excavated by a case auger whose small-diametered lower part is provided with a screw blade in such a way that no excavated soil is discharged and the wall ground of the pit is compacted.

CONSTITUTION: When the ground where a foundation pile is to be placed is excavated by a case auger A, soil and sand are pushed up by a screw blade 4 provided to the small diameter portion 1a at the lower part of the casing, part of the excavated soil and sand is compacted to some extent to the wall ground of the pit, and the soil and sand being pushed up are further compacted to the wall ground of the pit by the upper part 1b of the casing. Thus, the excavated soil and sand are partly stayed on the small diameter portion 1a and the remaining larger part of the soil and sand is compacted to the wall ground of the pit wherein being discharged to the ground's surface. When the excavation proceeds further until a given depth is reached, the auger A is turned reversely and then cement milk is injected through a nozzle 2 at the tip of the auger A, whereby the soil and sand remaining between the blades 4 go down and at the same time is disturbed and mixed with the cement milk injected by means of a disturbing blade 5 to form a soil mortar B. Then, the auger A is drawn up and a ready-made pile C is driven into.