PURPOSE: To make the structure of the positioner in the caption compact in its size and improve its accuracy and responsiveness by building a spool valve in a piston, and directly connecting the spool valve to a servomotor through a ball screw.

CONSTITUTION: A spool valve 3, which is built in a piston 2, is directly connected to the output shaft of a servomotor 9 through a ball screw 6 and a coupling 8. A balancing spring 5 is assembled in the upper portion of a cylinder main body 1, thereby the piston 2 is energized downward. The valve, a driving part and an actuator are united accordingly, so that the structure is made compact. In addition, since a feedback mechanical part is structurally simplified, the accuracy and responsiveness are improved.