PURPOSE: To detect fluid leakage early by completely filling a main tank with working fluid, forming an air bank in an upper part in an auxiliary tank and providing a fluid level detector in the 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 a plurality of communication ports 19, and the main tank section 14a is full of working fluid W, and an air bank A is formed at an upper part in the auxiliary tank section 14b, and the auxiliary 14b is provided with a fluid level detector 18 which detects the fluid levels W and L of the working fluid W. Because at the initial. stage of fluid leakage, the fluid levels W and L of the working fluid W in the auxiliary tank section 14b are suddenly lowered, the fluid level detector 18 is made to surely be activated according to the change of the fluid level to make it possible to detect fluid leakage early.