To automatically change a tool without rotating the tool while holding the tool by a holding part by synchronizing the rotation of the tool to that of a spindle when the tool is clamped to the spindle, or mounting the tool under rotation on a ring-shaped holding part when the tool is unclamped from the spindle, and rotating the tool in a synchronous manner, and then, unclamping the tool from the spindle.

A tool changing arm 2 is advanced, a rotating tool T is brought close to a ring-shaped holding part 3 of the tool changing arm 2, and a shoulder part of a spindle 10 pushes in a friction clutch 5. A spring 8 is thus deflected, a ball 4 is fitted in a groove of the tool T, and the holding part 3 is freely, rotatably and pivotably supported by the tool changing arm 2 through a bearing 7, and the tool T is rotated together with the spindle 10 by the urging force of the spring 8. By pushing a clamp rod 14 from an upper part, a clamp ball 16 is retracted to a recess 18, a pull stud 19 is disengaged, and at the same time, the tool changing arm 2 is retracted.