PURPOSE: To perform a highly precise center hole machining even when a lengthy work has a poor roundness by elastically installing a plurality of springs for energizing a drill holder having a drill fitted into its shaft center hole at a right angle to the shaft center line between the drill holder and a case for covering the drill holder.

CONSTITUTION: The base end 41 surface of a spring support shaft 40 is brought into contact with the end surface on four sides of a collar 23, and an aligning spring 39 is compressed between the base end 41 and a guide 44. The screwing of a nut 43 is regulated, whereby the shaft center line position of a drill holder 13 is determined. When the top end of a drill 15 makes contact with the end surface of a work, and a force for bending the top end of the drill 15 acts on it, the drill holder 13 is moved in the right-angled direction to the shaft center line as it is in the horizontal state against the aligning spring 39, and the drill 15 can be thus laid along a center hole.