PURPOSE: To make a smooth vertical movement of a weighted material and further make a small-sized driving mechanism by method wherein a weight balance of a cross rail, a sub-cross rail and a tracer head is adjusted by hanging down two hydraulic cylinders.

CONSTITUTION: The hydraulic cylinders 10 and 11 for use in hanging down the cross rail 3 are arranged at right and left sides. The saddle 2 and the head stock 1 are moved in a leftward direction on the cross rail 3, for example, resulting in increasing a load R1 applied on the left cylinder 11, decreasing a load R2 applied on the right cylinder 10 and generating a force for deforming and displacing the cross rail 3. In order to restrict the displacement and deformation of the cross rail, the plungers of the relief valves 14, 15 are moved vertically by cam plates 12, 13 having an oppositely faced inclination face, resulting in increasing a pressure of the left side hydraulic cylinder 11 and decreasing a pressure of the right side hydraulic cylinder 10.