A multi-spindle head exchangeable machine tool is provided with a plurality of multi spindle heads movably mounted on an annular rail. When a workpiece is machined by driving a rotary tool by a machining unit, multi spindle heads of the plurality of multi spindle heads being out of use can be separated from the machining unit. Further, a connecting device is provided with an inner slider, ball bearings fitted to an outer periphery of the inner slider, an outer slider, and a coil spring to press the outer slider toward a stopper member.