PURPOSE: To perform the positioning of a second tip part of an extension shaft and a tool mounted on this second tip part at an acute angle to an axial line of a chuck by forming the extension shaft so as to freely deform in all the directions of the axial line of the chuck.

CONSTITUTION: A first tip part 20 of an extension shaft 18 and a chuck 12 have complementarily non-circular cross sections, the extension shaft 18 is inserted into the chuck 12 in an axial line direction of the chuck 12 so that these cross sections may be revolved and matched and thus a first tip part of this shaft 18 is accepted by the chuck 12 and is held by a holding means. Further, a second tip part 22 of the extension shaft 18 and a tool 14 have a complementarily non-circular cross sections, this flexible extension shaft 18 is normally arranged so as to be almost concentric with the chuck 12 as a whole, the shaft 18 properly deforms in all the directions of the axial line of the chuck 12 and the positioning of the tool 14 held by this chuck 12 at the second tip part 22 is performed at an acute angle to the axial line of the chuck 12.