PURPOSE: To control the direction of a drill head using high pressure fluid by setting diameters of an orifice, a chamber and a rod of a drilling/steering assembly, and transferring a nose member and a steering part in response to a specified pressure change.

CONSTITUTION: A drilling/steering assembly 20 is formed by a nose member 30 and a steering part 40. A pair of nozzles 35A, 35B are provided at the tip of a fluid passage 33 piercing the nose member 30, insert members 36, 37 are provided in the inside, and a hollow push rod 50 is provided on the steering part 40. According to the orifice diameter of the insert members 36, 37, only a fluid of a flow corresponding to specified pressure is jetted from the nozzles 35A, 35B, and when the fluid pressure is further increased, the travel of a piston 55 in a chamber 47 is increased so that the push rod 50 and also the nose member 30 and the steering part 40 are transferred like a circular arc. Thus, the drill head can be controlled to a desired direction using high pressure fluid.