PURPOSE: To simplify a circuit construction by installing a double acting auxiliary hydraulic cylinder in parallel with a hydraulic cylinder controlled with a control valve for the horizontal control.

CONSTITUTION: A double acting auxiliary hydraulic cylinder 5 is installed in parallel with and separate from a hydraulic cylinder 4 controlled with a control valve 3 for the horizontal control and either of the hydraulic cylinders 4 and 5 is alternatively operated. Thereby, the formation of a special hydraulic circuit for expansion and contraction control of the auxiliary hydraulic cylinder is not required.