To prevent a lowering of the verticality of a bore in a drilling step of an earth drill method.

In the earth drill device, mount bars 32 are attached to the upper part of a bucket 6 and buoyant bodies 31 are mounted on the top ends of the bars 32. If a thrust 33 of a kelly bar acts in a direction biased from the center of a bore 2, a buoyant force acts on the body 31 in a stabilization liquid 11 so that the bucket 6 is lifted in the vertical direction. The buoyant force generates a restoring force 34 vertical to the direction of the thrust 33 of the kelly bar so that a rotation force 35 is applied to the bucket 6 and hence a restoring force 36 is generated owing to the thrust 33 of the kelly bar, which force 36 corrects the inclination of the bucket 6 to maintain the verticality of the bore.