To increase efficiency in excavation by forming a boring rod as a double tube structure forming a slime discharge space, and arranging a gathering part in the middle of slime discharge passages.

When a boring rod is driven into the ground, an outer tube 3 and an inner tube 2 are threadedly engaged together, and are connected to each other in a multistage shape, and are deepely driven into the ground. When a rotary hammering device is actuated, a bit edge surface 31 of the boring rod rotates, and the inside of the ground is excavated. At this time, since high pressure water or air is injected into excavation, pressure is applied to excavated slime 30, and the slime 30 is discharged to a space 32 between the outer tube 3 and the inner tube 2 through discharge passages 28 formed on an outer peripheral surface of an inner bit 4. A gathering part 29 to communicate with the respective discharge passages 28 is formed in the middle of the discharge passages 28, and even if slime 30 intensively enters the discharge passages 28, it is temporarily discharged in the gathering part 29 in the middle, and resistance applied to the slime 30 is eliminated, and a flow in the discharge passages 28 is smoothed.