PURPOSE: To position a spool and so forth correctly without any effect by a secular change by installing a cylinder mechanism provided with a piston rod expandable according to the volume change in a container, the volume modification member of the container and the volume change of the container.

CONSTITUTION: A positioning actuator is provided with a container 1 where hydraulic oil S is sealed, a volume modification rod 2 where the volume of the container 1 can be changed and a cylinder mechanism 5 equipped with a piston rod 8 which is expandable according to the volume change of the container 1 and is connected in serial to the inside of the container 1. With such a means, the protrusion amount of the piston rod 8 can be determined in multiple stages and correctly according to the volume change in the container 1. When the positioning operation of an object to be controlled such as the spool of a proportional solenoid flow control valve is controlled, for example, it is difficult to change the position of the piston rod 8 even if the traveling resistance of the object to be controlled varies with a secular change or fluid power working upon the object to be controlled varies, thus positioning the object to be controlled can stably be made at a prescribed multi-stage position.