To reduce noises without reference to boring attended by percussion and adapt to any types of object grounds etc.

A hydraulic hammer 10 for operating a striking bit 40 at its tip under the supply of high-pressure water to a built-in striking mechanism 42 is integrally connected to the front end part of an inner rod 18, and coaxially inserted into the front end part of a casing pipe 14. A rotating force is imparted to each of the casing pipe 14 and the inner rod 18 by a rotary drive unit which is arranged at each back end, and a striking force is imparted to the striking bit 40 of the hydraulic hammer 10 under the supply of the high-pressure water from a high-pressure pump to the striking mechanism 42 via a channel 16 inside the inner rod 18, so that a boring thrust force by a double boring pipe 20 can be secured. In addition, the drainage of the high-pressure water, which is supplied to the striking mechanism 42 of the hydraulic hammer 10, is sequentially discharged for circulation into a bore hole 54, which is bored by the pipe 20, from a drainage hole 40b which is provided in the striking bit 40.