To provide a servo cylinder reducing total length as compared with conventional one.

An electrohydraulic servo cylinder 100 is provided with: an electric motor 110 rotating a driving shaft 111; a cylinder 121; a piston 122; a driving part 120 including a ball screw 123 rotating in proportion to movement of the piston 122: and a spool valve 130 having an internal screw 132a meshing with an external screw 111a formed on a casing 131 and a drive shaft 111 formed thereon, having teeth 132b meshing with teeth 123a formed on outer circumference of the ball screw 123, including a helical gear 132 rotating and moving and a spool 133 rotating and moving together with the helical gear 132, and feeding and discharging hydraulic fluid in proportion to movement of the spool 133 to and from a cylinder chamber 120a and a cylinder chamber 120b. A rotating shaft of the ball screw 123 and a rotating shaft of the helical gear 132 are arranged in such a manner as to cross at a right angle.