PURPOSE: To prevent the occurrence of working fluid separation phenomenon by completely filling a main tank with working fluid and forming an air bank in an upper part in an auxiliary tank.

CONSTITUTION: The working fluid tank 14 attached to an actuator 12 comprises a large long sideways rectangular parallelepiped main tank section 14a, a small short sideways rectangular parallelepiped auxiliary tank section 14b, and both the tank sections 14a and 14b are communicated to each other through one or a plurality of communication ports 18, and and an air bank A is formed at an upper part in the auxiliary tank section 14b. Because the air bank A in the auxiliary tank 14b is small and the surface area of the fluid surfaces W and L at their levels of working fluid W are small, the quantity of the water vaporized from the fluid surfaces is small, as a result, the separation phenomenon of the working fluid W hardly occurs, which makes it possible to surely activate the actuator 12.