To obviate the necessity of excessively enlarging the constitution for undertaking propulsion reaction forces and rotational reaction forces, and avoid limiting the propulsion forces and rotational forces in the period of excavation by the strength or dead weight of the constitution undertaking the propulsion reaction forces and rotational reaction forces, furthermore, withdraw excavating machine outside of the foundation shaft readily after the completion of excavation.

In the excavating period, grippers 25 are spread out toward the foundation pile by means of gripper jacks 24 so that the excavator main body 20 is supported on the foundation pile. Then, a cutter 28 is rotated by a activator 30 in order that it is pushed against rocks by means of propulsion jacks 27 to thereby excavate then. After the completion of excavation to the bottom of the foundation shaft, the cutter blades 31 are pulled back by the expansion jacks 33 to contract the cutter 28 smaller than the inner diameter of the exterior frame 21, then the excavator main body 20 is removed outside of the foundation shaft from the exterior frame 21.