PURPOSE: To drill a large hole and to perform smooth excavation of a hard ground, by a method wherein the drill bit of a drilling device, attached to an excavating shaft, is disposed to the lower end of the excavating shaft, and the axis of the drill bit is positioned eccentrically from the axis of the excavating shaft.

CONSTITUTION: When excavating blades are arrested by the hard ground of a bedrock, smooth excavation is not permitted by means of excavating blades 2, but the ground is drilled by a drill bit 4 through driving of the drill bit 4 of a drilling device 3 and is further excavated by means of the excavating blades 2. Thereafter, when the ground is drilled by the drill bit 4, a drill shaft 1 is rotationally driven, and the drill bit 4, positioned eccentrically from the axis of an excavating shaft 1, performs drill as it is rotationally driven to excavate a large hole. The formed hole is scratched by scratching blades 2a to provide an increased size to procede with excavation. This permits excavation of a large size hole through drilling, and enables further smooth excavation of the hard ground.