Hydraulic screw-down device used in small-size rolling mill or roll press machine and hydraulic control method using the same

In a small-size rolling mill or a roll press machine, to provide a hydraulic screw-down device compatible with the plate thickness accuracy without using a hydraulic control servo valve.A hydraulic screw-down device that is comprised of a screw to move a piston arranged in a booster cylinder back and forth, and a motor that gives rotational force to the screw controlling freely the rotational angle of the screw so as to move the piston to the purpose position. A screw-down cylinder connected with the booster cylinder through piping makes a ram in the screw-down cylinder move vertically by the move of oil caused by the movement of the piston in the booster cylinder and thereby enables push-up and push-down of the mill roll.