PURPOSE: To utilize a conventional fluid circuit even under environment of high or low temperature by closing a servo control valve when a piston is positioned at a desired position, and opening it when a piston is positioned at a locked position.

CONSTITUTION: When a servo actuator 1 is utilized under environment of high or low temperature, temperature of the actuator 1 becomes similar to that of the outer environment, some problems such as deterioration of packing and raising of fluid viscosity occur. In order to solve these problems, a first chamber 4a is communicated with a second chamber 4b by forming a throttle passage 10 in a piston 3. Fluid having high pressure leaks through a small void formed on a servo control valve 14, but the fluid flows out to the lower pressure side chamber 4a or the chamber 4b because the passage 10 is formed on the piston 3 after the fluid flows into the chamber 4a or 4b via the passage 10. Thus, although the servo control valve is closed, small amount of fluid flow is formed in the actuator 1. Therefore, the actuator 1 is cooled or heated with a certain temperature by this fluid.