PURPOSE: To enable change-over between the maximum discharge quantity and the maximum discharge pressure according to the loaded state in a liquid plunger type booster.

CONSTITUTION: A cylindrical auxiliary cylinder chamber 9 is formed at a plunger 3 inserted into a cylinder chamber 2 in a cylinder 1. An auxiliary plunger 10 fixed to the cylinder 1 is inserted in the freely advancing/receding state into the auxiliary cylinder chamber 9 to form an auxiliary pressure chamber 11. A check valve 13 for inhibiting contraflow to a main pressure chamber 4 in the cylinder chamber 2 is provided at an inlet combined discharge port 12 for communicating the main pressure chamber 4, the auxiliary pressure chamber 11 and a discharge passage 7. A valve force-opening means 14 is further provided to force-open a suction valve 6 in a suction passage 5 when the discharge pressure from the discharge passage 7 is the specified value or more.