PURPOSE: To efficiently bore a round hole in a lithic material by providing a main shaft so as to arbitrarily and freely raise and lower the same by a manual handle and a lift cylinder.

CONSTITUTION: A core drill is allowed to fall by a manual handle 4 until brought into contact with a lithic material and further sent little by little until it bites in the lithic material. Next, the dog of a cutting-in depth gauge is adjusted so as to be matched with hole depth and the selector switch of a compressed air passage is changed over to falling to allow a lift cylinder 5 to fall and, thereafter, a main shaft and an outer cylinder are allowed to fall by the lift cylinder 5 having a spring like element. When the core drill reaches hole depth, a position sensor is operated by the dog of the cutting-in depth gauge and, therefore, the lift cylinder 5 is finely moved up and down several times by the difference between the velocity of the back pressure air stream of the lift cylinder 5 and exhaust flow velocity to improve the outflow of chips.