To enable an auger forward end to stably bite a subsoil and accurately excavate a hole at an expected position by applying the constitution that the external surface of an auger is provided with an excavating casing having a size longer that the auger forward end by a preset size or above and having a rotating function for excavation.

An excavator 1 is suspended from a crane truck 6 and an auger 2 is held with a leader 5 for travel in a long-sized direction. The external surface of the auger 2 is provided with a casing 3 rotating for excavating a subsoil, and an auger forward end 2a is set at a position 5 cm or longer inward from the forward end of the casing 3. In this case, at least the forward end part of the auger 2 and/or the forward end part of the casing 3 is made of a cemented tip having a shock resistance function, and an overlay having a wear resistance function is formed on the external surface of an area above the forward end part. The overlay is applied at a construction site, thereby easily making a compensation for wear. Positioning is made by use of a guide rule 4 and the forward end of an auger 2 while rotating is first made to bite a subsoil. At the same time, the forward end of an auger 2a is guided to an expected position for stabilizing the biting of a subsoil, thereby implementing accurate excavation.