PURPOSE: To aim at the shortening of tool exchange time, by reciprocating a screw shaft through two concentric cylinders, and turning a change arm by rotation of a change nut via an arm main shaft.

CONSTITUTION: Small rotation of a tool exchange arm 1 is carried out by what a cylinder rod 14 of a short cylinder 13 is made to go down, while a change nut 10 is rotated by a screw shaft 12, and a spindle 2 is rotated via an outer shaft 5. In this case, a stroke of the short cylinder 13 should be set as indexed from a turning angle of the tool exchange arm 1. Next, as for large rotation of the tool exchange arm 1, it takes place with this exchange arm 1 rotated as far as 180 degrees by the spindle 2 with the lowering of a cylinder rod 16 of a long cylinder 15 via the screw shaft 12 and the change nut 10. Thus, a driver can be compactly assembled within a projection area, and tool exchange time is made reducible in consequence.