To boost a low pressure to a high pressure by a simple mechanism without requiring other power sources such as a hydraulic motor and a hydraulic pump.

When a spool 2 is advanced by applying the liquid pressure introduced from outside to a pressure receiving mechanism 3 for advance, the liquid in a pressure receiving mechanism 5 for retreat is compressed by the advancing action of the spool 2, and simultaneously with this, the spool 2 is retracted at a suitable position by a switching mechanism 6 by controlling the energizing force of the pressure receiving mechanism 3. When the spool 2 is retracted, the action of functioning the pressure receiving mechanism 3 is repeated by utilizing the switching mechanism 6 again in order for the spool 2 to reciprocate to take out the boosted liquid pressure from the pressure receiving mechanism 5. Thus, the effective boosting function can be performed by a simple structure by utilizing the liquid itself requiring boosting as a power source.