To provide a servo jack device used by both of screw and liquid pressure, operating by a motor, and used in a press machine requiring high speed by low thrust and high thrust at low speed without using a power unit composed of a pump, a control valve, etc. by combining a screw type cylinder and a liquid pressure type cylinder mutually to prolong its service life, simplify its structure, and reduce its cost.

This servo jack device is composed of a main body 2, a motor 3, a screw bar 4, a screw plate 5, a first piston 6, a first cylinder 7, a first liquid pressure chamber 8, a second piston 9, a second cylinder 10, a second liquid pressure chamber 11, a third liquid pressure chamber 12, a first passage 13, a second passage 14, and a selector valve means 15. In particular, this device is provided with the selector valve means 15 for opening and closing the first passage 13 and the second passage 14 selectively. That is, the first piston 6 and the first cylinder 7 can be locked by liquid pressure to dispense with a conventional mechanical connection mechanism.