To actuate just by supplying a working fluid from outside and using a fluid pressure of the working fluid as a power source, and to provide a simple constitution which is capable of continuous output.

An advance pressure receiving mechanism 3 and a retreat pressure receiving mechanism 5 which affect a spool 2 engaged into a housing 1 where it can advance and retreat, with an advance energizing force and a retreat energizing force due to introduction of a working fluid from outside, are provided. And also provided are first and second high pressure generating parts 6 and 7 which compressively high-pressurize the working fluid introduced inside using the advance or retreat energizing force. In such a case, accompanying the advance and retreat of the spool 2, a compression stroke is performed by one of the high pressure generating parts 6 or 7 and the other performs an intake stroke. At the same time, the advancing and retreating direction of the spool 2 is automatically changed at advance and retreat positions by a switching mechanism 8 so as to constantly compressively high pressurize the working fluid at either of the high pressure generating parts 6 or 7, and a priority take out mechanism 9 selectively takes out the high pressurized working fluid.