To automatically perform a process from rough machining to a spherical finish by applying the constitution that a machine tool used for boring or rough machining bore internal surface is provided with a sphere machining device for finishing a workpiece to a sphere as integral part thereof.

Bore internal surface is rough machined with a tool T under the axial motion of a column 48 and a tool rest 47 along an Z-axis and a X- axis. Then, the column 48 and the tool rest 47 are axially moved along a Z-axis and the X-axis, thereby being made to retreat from a workpiece W. Also, a sphere machining device 1 is axially moved in the Z-axis direction along a horizontal guide 46, thereby inserting a cutter 16 in the bore of the workpiece W. Then, the workpiece W is rotated, together with a main spindle 42, and a cutter 16 attached to a rotor 4 is rotated, thereby finishing bore internal surface Wa to a sphere.