PURPOSE: To perform boosting in a state to maintain the degree of cleanliness at a high value without polluting treatment liquid by causing conduction of a piston on the secondary side between cylinder chambers on the primary and secondary sides and forming a cleaning chamber to contain and remove a leaking pressure medium in the cylinder chamber on the primary side.

CONSTITUTION: When pressure medium oil is introduced through a content liquid entrance 2 on the primary side to a cylinder chamber 1 on the primary side, pistons 3 and 4 on the primary and secondary sides are orderly operated. The pressure of containing liquid in a cylinder chamber 5 on the secondary side is increased and delivered through an outlet 6 to a treating tank. In this case, when pressure medium oil in the cylinder chamber 1 on the primary side leaks, a leakage detector 11 in a cleaning chamber 9 is operated to release a control valve 14. After a pump 15 is operated and washing liquid in a tank 16 is fed with a pressure through an introduction port 12 to the cleaning chamber 9, the washing liquid is recovered through a discharge port 13 to a recovery tank 18 by means of a piping 17. As a result, even when a pressure medium in the cylinder chamber 1 on the primary side leaks, it can be removed in the cleaning chamber 9, whereby treatment liquid in the cylinder chamber 5 on the secondary side is prevented from being polluted but its pressure can be increased.