PURPOSE: To facilitate manufacture of a pressure device by engaging a stepped piston comprising a slender shaft, a thick shaft and a slender shaft arranged in order with a drum portion comprising a narrow piston chamber and broad piston chambers.

CONSTITUTION: A drum portion 13 comprises a narrow piston chamber A dis posed on cylinder bead 11 side and broad piston chambers (a), B disposed on the other end side. A piston of a step structure has a slender shaft 21, the outside diameter of which is engaged with the inside diameter of the narrow piston chamber A of the drum portion 13 so as to be able to take piston action. Secondly a thick shaft 22 is provided, with the outside diameter thereof engaged with the inside diameters of the broad piston chambers (a), B. Lastly there is provided a slender shaft 23 where the output side of the piston action is engaged with a shaft hole of a flange 14 so as to be able to take piston action.