A machine tool comprising: a table configured to turn about a vertical axis (C 2) with a workpiece mounted on the table; a spindle head disposed above the table and pivotable about a B-axis, the B-axis is a horizontal axis; a spindle built in the spindle head and configured to rotate about an axis of a tool detachably mounted to the spindle; and an internal turning attachment detachably mounted to the spindle head with a dedicated clamp mechanism (64, 212), wherein the internal turning attachment comprises an attachment base having a portion to be mounted to the spindle head, a body fixed to a lower portion of the attachment base, and a tool holder to be mounted to a lower portion of the body, wherein the tool holder has a tool mount surface to which a turning tool is detachably mounted, wherein a tool mount angle ( β2) is an angle between the tool mount surface and a plane perpendicular to an axis (C 0) of the spindle as the internal turning attachment is mounted to the spindle head, characterized in that the tool mount angle ( β2) is changeable, for a workpiece which has a hole with a depth deeper than a length of the internal turning attachment projected from the spindle head, the machine tool is configured to change the tool mount angle ( β2) larger than zero degree and to change an angle of the spindle head about the B-axis to other than zero degree, and is configured to machine the workpiece at the changed tool mount angle ( β 2), and the machine tool is configured to fix the changed tool mount angle ( β2) while the turning tool is in contact with the workpiece. a table configured to turn about a vertical axis (C 2) with a workpiece mounted on the table; a spindle head disposed above the table and pivotable about a B-axis, the B-axis is a horizontal axis; a spindle built in the spindle head and configured to rotate about an axis of a tool detachably mounted to the spindle; and an internal turning attachment detachably mounted to the spindle head with a dedicated clamp mechanism (64, 212), wherein the internal turning attachment comprises an attachment base having a portion to be mounted to the spindle head, a body fixed to a lower portion of the attachment base, and a tool holder to be mounted to a lower portion of the body, an attachment base having a portion to be mounted to the spindle head, a body fixed to a lower portion of the attachment base, and a tool holder to be mounted to a lower portion of the body, wherein the tool holder has a tool mount surface to which a turning tool is detachably mounted, wherein a tool mount angle ( β2) is an angle between the tool mount surface and a plane perpendicular to an axis (C 0) of the spindle as the internal turning attachment is mounted to the spindle head, characterized in that the tool mount angle ( β2) is changeable, for a workpiece which has a hole with a depth deeper than a length of the internal turning attachment projected from the spindle head, the machine tool is configured to change the tool mount angle ( β2) larger than zero degree and to change an angle of the spindle head about the B-axis to other than zero degree, and is configured to machine the workpiece at the changed tool mount angle ( β 2), and the machine tool is configured to fix the changed tool mount angle ( β2) while the turning tool is in contact with the workpiece.