When a main spindle (10) is rotated and the main spindle and a work are moved linearly and relative to each other in the axial direction (Z) of the main spindle in, for example, the threading of a machine tool, a spindle servo-motor (12) and a feed servo-motor (22) as the driving sources must be actuated while they are being synchronized. Generally, the spindle servo-motor drives directly the main spindle without resort to a reduction gear but the feed servo-motor drives a main spindle head (18) through reduction gears (26, 28). Therefore, the synchronization is generally governed by the acceleration/deceleration performance of the spindle servo-motor. The synchronization with maximum efficiency can be accomplished by accelerating and decelerating the main spindle while actuating the spindle servo-motor in line with its torque limit curves (L1, L2).