To provide a multistage booster capable of setting the pressure of a secondary side at plural sages with one booster and capable of supplying the various high pressure in response to a load.

In this multistage booster included in a fluid, oil for example, pressure circuit, a piston-plunger member to be driven by a position 5, to which the pressure of the working fluid of a primary side is to be applied, is arranged inside of plural cylinders 2, 3, and 4 continued in order of shrinkage of the inside cross section thereof. The cylinders 2, 3 and 4 are connected to a load side such as an actuator 11 through a fluid flow passage, and each fluid flow passage is provided with control valves 8a, 9a and 10a for controlling opening and closing of the flow passage to the load side.