To provide a booster capable of operating with using a designated pressure of hydraulic fluid itself as a power source by just only supplying hydraulic fluid from an outside, capable of continuously outputting, small in size, capable of utilizing pressure of hydraulic pressure efficiently, and easy for processing.

Pressure receiving mechanisms 5, 6 applying forward and retreat energizing force to a spool 2 are provided at a front part and a rear part of a housing 1 separately. That allows providing each one of a front part fluid path 9 and a rear part fluid path 8 in series in the spool 2. Consequently, simplifying a structure of the spool 2 reduces size of the spool 2 and housing 1 of a booster. Enlargement of both of fluid paths 8, 9 eliminates pressure loss of hydraulic fluid to efficiently utilize pressure of hydraulic fluid. Processing of the spool 2 is also simplified.