PURPOSE: To lengthen the working service life by separating a part of an inflow fluid containing no drilling object as a working medium of a hydraulic system, and preventing a polluted drill fluid from flowing in a housing.

CONSTITUTION: A rotor of a generator 23 rotates together with a drill shaft 2 to generate electric energy. A pressure pulse generator 25 is driven by a clean drill rod fluid, a hydraulic working pressure is derived, control plates 11 to 14 are swung, and a drill rod 1 is held in the prescribed drill direction. A partial flow of the clean drill rod fluid is separated into an annular space 26 to prevent a polluted drill fluid from flowing in a housing 10 from a drill hole. Both ends of the annular space 26 are sealed by rotation check valves 28, 29 from a boring fluid flowing between the housing 10 and a boring surface.