Abstract: In an oil-well-pump driving hydraulic system capable of avoiding occurrence of cavitation, at least a bidirectional variable displacement piston pump, a discharge rate detection portion, a discharge pressure detection portion, and a proportional solenoid control valve are contained as a single unit structure in a same housing as a pump body. The pump body includes: a connection portion provided on a side surface of the housing and to which a pump discharge passage of an external pilot hydraulic circuit is connected; and a second pilot passage that introduces an external pilot pressure from the connection portion to a pilot passage between the discharge passage of the piston pump and the proportional solenoid control valve.

Abstract: In an oil-well-pump driving hydraulic system capable of avoiding occurrence of cavitation, at least a bidirectional variable displacement piston pump, a discharge rate detection portion, a discharge pressure detection portion, and a proportional solenoid control valve are contained as a single unit structure in a same housing as a pump body. The pump body includes: a connection portion provided on a side surface of the housing and to which a pump discharge passage of an external pilot hydraulic circuit is connected; and a second pilot passage that introduces an external pilot pressure from the connection portion to a pilot passage between the discharge passage of the piston pump and the proportional solenoid control valve.