PURPOSE: To perform kelly dig boring operations easily by adjusting the sinking speed of a kelly rod with a simple mechanism.

CONSTITUTION: A kelly balance device 31 includes a drum 32, gear 33, transmission 34, and hydraulic motor 35, wherein the gear 35 transmits rotation of the drum 32 to the hydraulic motor 35. One end of a wire rope 36 hanging a digging tool is wound fast on a drum 23 of a boring machine 20, while the other end is fast on the abovementioned drum 32 of kelly balance device 31. The sinking speed of the digging tool including kelly is adjusted by a throttle valve (55), when the drum 23 of the boring machine 20 shall be kept at a standstill by brakes. The kelly balance device 31 requires only connection to the oil-hydraulic circuit of the boring machine 2O through two hydraulic hoses 37. Accordingly the kelly dig boring operations can be performed without installing any hydraulic brake device in the boring machine or subjecting it to a modification of large scale.