To save energy more by reducing power consumption of an electric motor which rotationally drives a pump and by lowering a discharge pressure of the pump which discharges an operating fluid introduced into a booster cylinder.

When the operating fluid which is discharged from the pump 2 is introduced into the inside of a small cylinder part 7, a head side chamber 15 of the booster cylinder 5 introduces compressed air so that pressure of the compressed air will work on an other face of the piston 13 and at the same time, an electromagnetic change-over valve 23 is provided for release to the atmosphere switchably when the operating fluid filled inside the small cylinder part 7 is discharged. Thereby, an acting force based on the pressure of the compressed air which acts on the other face of the piston 13 is converted into a pressing force which pushes the piston rod 12 and the piston 13 to a large cylinder part 6 side, the discharge pressure of the pump 2 can be reduced by a quantity corresponding to the force, and more energy can be saved by reducing power consumption of the electric motor 3.