PURPOSE: To widen the range of absorbable and correctable errors of positioning by a robot as well as to enhance accuracy for drilling and countersinking by making it possible to lock the flange of a drill bush onto the locked section of a drill plate with a nose piece revolved.
CONSTITUTION: A first and a second lock mechanism 31 and 37 are locked, and the tip end of the drill bush 21 of a drilling unit 3 is moved to a position just above a drill position. In the second place, a nose piece 20 is lowered down by a nose piece inserting actuator 39, the first and second lock mechanisms 31 and 37 are released after the tip end of the drill bush 21 comes in contact with the top section of the guide bush 23 of a drill plate 22, the drill bush 21 is inserted into the guide bush 23, the nose piece 20 is revolved by a nose piece revolving mechanism 27 with the first lock mechanism 31 locked, and the flange of the drill bush is locked onto the locked section of the drill plate 22, so that the drill bush 21 is thereby fixed. Then, the lock of the first lock mechanism 31 is released, both of the lock mechanisms are thereby turned into an unlocked condition at the time of drilling and countersinking, and both a drilling unit robot and the drill plate are thereby never subjected to unreasonable bending stress.