PURPOSE: To hasten the speed of excavation for great deep excavation by a method in which a turning force and an impact force are given to an outer casing having a bit on its tip and a hollow inner rod from the ground's surface for small-deep excavation range.

CONSTITUTION: A fresh water supply source 26 is connected through pump to an injection port 11, and a driver 6 is operated to give turning force and impact force to both an outer casing 1 and an inner rod 8. The ground is excavated by bits 4 and 16 and fresh water is supplied from the source 26 and discharged from near the bit 16 through the flow path 14 of the rod 8. As the ground is excavated by the bits 4 and 16, the water as mud water is discharged to the ground's surface through the drain port 12. For deeper excavation, the source 26 is connected to a pressure air supply source 27. Pressure air is supplied to an air hammer 15 to give an impact force to the bit 16 by the striking force of the hammer 15, where the forces of the driver 6 and the hammer 15 are absorbed by an impact absorbing mechanism 17. The excavating speed for great-deep excavation can thus be sufficiently increased.