Turning machine and machining method by the same

A turning machine comprising:

• a main spindle including a chuck which grasps a workpiece;
• a tool-holding unit on which tools are mounted for machining the workpiece;
• a guide bush which supports slidably the workpiece;
• a measuring device which measures a distance between an immobile reference point which is immobile in a first direction, perpendicular to an axial direction of the workpiece and a variable point in which varies so as to be linked to the tools that is displaced with a progress of machining of the workpiece;
• and
• a tool-holding unit driving device which moves the tool-holding unit together with the tools in the first direction based on measured distance by the measuring device between the reference point and the variable point so that center height of the tool and a center line of the workpiece satisfy predetermined positional correlation;
• wherein the reference point is set on the external surface of the guide bush, facing the variable point in the first direction, and the measuring device is fixed to the tool-holding unit at a position facing to the reference point, and the variable point is set on the measuring device, while the measuring device measures a distance along the first direction between itself and the reference point.