To use even for a tool requiring a large amount of operating oil for a pre-load, by selectively connecting, an auxiliary oil supply passage to the front stage return oil passage of a low pressure relief valve, the rear stage return oil passage thereof or a drain oil passage through a selector valve.

A low pressure relief valve 19 is interposed in a return oil passage 8, and a selector valve 20 is interposed among the front stage return oil passage 8 of the low pressure relieve valve 19, the rear stage return oil passage 8 thereof, or an auxiliary oil supply passage 15. The selector valve 20 is changed over to a switching position (a), and the auxiliary oil supply passage 15 is connected to the front stage return oil passage 8 of the low pressure relief valve 19. Hereupon, the driving hydraulic pressure of a driving cylinder A, controlled by the low pressure relief valve 19, is led to flow into the compression chamber 13 of a pressure intensifying cylinder B through the auxiliary oil supply passage 15 and a check valve 14, and a tool 18 is pressurized by this inflow hydraulic pressure. Therefore, this booster can be used even for the tool 18 requiring a large amount of operating oil for pre-load.