To increase a distance between a drill machining head and a bush plate as much as possible during a tooling change while shortening a guide bar without being restricted by the length of the guide bar.

During replacing of a drill 14 or a guide bush 16 by another, a locking pin 37 of a locking means 41 is engaged with a locking groove 35 while a drill machining head 7A is in the advanced condition. Simultaneously, the guiding pin 20 of a guide bar pulling-out preventing means 40 is disengaged from a guiding groove 19. In this condition, while a bush plate 15 is left together with a guide bar 18 in a machining jig 6 side, the drill machining head 7A is retreated to an normal use retreating limit position P2.