To prevent inclination of a cross rail by rotating a screw shaft with displacement quantity of the screw shaft of a value at the time when a main shaft head moves against a standard value when it is at a central position of the cross rail as quantity to be corrected by detecting a position of a shaft end surface on the opposite load side of the screw shaft for cross rail elevation.

In the case when a main shaft head moves left, a load applied on a ball screw 8 on the right side becomes small, a load applied on a ball screw shaft 16 on the left side becomes large, and corresponding nuts 11, 18 are lowered. Displacement quantity of the nuts 11, 18 appears as displacement quantity of shaft end surfaces 9, 17. Consequently, displacement of the shaft end surfaces 9, 17 is measured, these values W1, W2 are input as corrected values to a command value W of servo motors 13, 20 as corrected values, and supposing corrected command values are W+W1, W+W2, the nuts 11, 18 are displaced, and the main shaft head 2 returns to a position in the center of the cross rail 1.