To hold shafts of respective drilling rods in a fixed drilling direction so as to make correct ground excavation possible by journaling the respective drilling rods which are rotationally driven and journaled on the ground, with each other under the ground.

A drilling rod 11 of minimum diameter precedingly drilling a small diameter hole A in the ground and a drilling rod 12 of maximum diameter drilling a spread large diameter hole B in the ground following to the drilling rod 11 are arranged into a double tube form. The drilling rod 11 of minimum diameter is rotationally driven in the state journaled with the drilling rod 12 of maximum diameter, and the preceding hole and the small diameter hole A are drilled. Next the drilling rod 12 of maximum diameter is rotationally driven in the state journaled with the drilling rod 11 of minimum diameter, and the large diameter hole B is drilled. The respective rods 11, 12 are mutually journaled with the other drilling rods 11, 12 not only on the ground but under the ground, the shafts of the respective drilling rods 11, 12 are held in the fixed direction, and hence correct ground excavation can be made possible.