PURPOSE: To cope with the large stroke of an input rod by arranging oil passages for feeding and discharging oil in the axial direction in a piston part and oil passages for feeding and discharging oil in the axial direction of its input spool rod in a circumferentially deviated manner so as not to interfere with each other.

CONSTITUTION: A groove for feeding oil to a rear oil chamber and a groove for discharging oil from a forward oil chamber are provided on the surface of an input rod 20 at a rear part in the axial direction, a groove 8 for feeding oil to the forward oil chamber and a groove 9 for discharging oil from the rear oil chamber are provided at the center part in the axial direction, and a passage 32 for discharging oil is provided at the forward part in the axial direction by forming the angle of phase of 90° in the circumferential direction so as not to interfere with each other. An oil feeding chamber 17 is provided on the center and outer circumference of a piston 23 of a piston rod 22, to communicate the oil feeding chamber 17 with the groove 8 for feeding oil to the forward oil chamber through a port 3 for feeding oil to the forward groove, the groove 8 for feeding oil to the forward oil chamber with a forward oil chamber 11 through a port 2 for feeding oil to the forward oil chamber, the groove for discharging oil from the forward oil chamber with the forward oil chamber 11 through a port for discharging oil through a port 5 for discharging oil from the forward oil chamber, and the groove for discharging oil from the forward oil chamber with a center port 30 through the forward oil discharging port. The oil grooves are not interfered with each other even when they are long, and a servo motor of large stroke is obtained.