Abstract: The rotation angle around x axis, inclination angle and directional angle of a boring bit are obtained from the value measured with an accelerometer (22) and a three axis magnetic sensor integrally provided within the boring bit attached to the top of a rod array for boring. From these obtained values and the length corresponding to a rod, the amount of varied position according to boring by a length corresponding to a rod is obtained for each directional component. Further, the position of boring corresponding to boring in plural times with successive addition of rods is obtained through accumulating the amount of varied position for each directional component in respect of boring in plural times.