Locking elements 3d and 3j are provided at an inner member 3 connected to a kelly bar 1, and a locking element bearing plate 6f is provided at an outer member 6. The outer member 6 includes a cylindrical bucket 7 and a grab bucket 8 housed inside the cylindrical bucket 7. When an excavating tool 2 is in its most contracted state, the inner member 3 is rotated forward to lock the locking elements 3d and 3j at the locking element bearing plate 6f, thereby disallowing relative vertical movement of the inner member 3 and the outer member 6. As the excavating tool is rotated by applying a force to the kelly bar 1 along the lifting direction in this state, an excavating operation can be executed while applying a load smaller than the load of the excavating tool 2. As a result, it becomes possible to execute an excavating operation with a large excavating tool in conjunction with an earth drill having a small drive force. Projections 3k provided at the outer circumference of the inner member 3 are fitted at guide rails extending along the longitudinal direction and provided at the inner circumference of the second member 5 so as to be allowed to move up/down freely. Thus, the grab bucket 8 having an underground obstacle grabbed therein can be rotated to remove the underground obstacle. <IMAGE>