A position ensuring system includes an A-axis calibration system which measures a displacement angle, which is an error between a target value and a measured value of the pivot angle of the spindle head about the A-axis, and corrects the pivot angle about the A-axis in such a manner that the displacement angle as measured with the corrected pivot angle as a target value fall within a tolerable range. A corrected data storage device stores the corrected pivot angle about the A-axis. An A-axis control system reads out the corrected pivot angle about the A-axis, the corrected pivot angle about the A-axis to pivot the spindle head when executing oblique machining of the inclined hole.