A fluid pressure producing method for pressuring a fluid using a plunger (261, 262) driven by operating a double-acting piston with a pressure of a working medium produced from a working medium pump, the method comprising: switching a moving direction of the piston when detecting the piston reaching a movement end; detecting a load (Tr) of the working medium pump and determining whether the detected load is not less than a load limit (TrL); operating the working medium pump to discharge the working medium at a maximum flow rate while the load (Tr) is less than the load limit (TrL) after the moving direction of the piston is switched; and detecting a pressure of the fluid and feeding back the detected pressure of the fluid and performing feedback control of the pressure of the fluid to eliminate a difference between the pressure of the fluid and a target pressure of the fluid (P2com), characterized in that the feedback control of the pressure of the fluid is performed after the load (Tr) reaches or exceeds the load limit (TrL), and the feedback control of the pressure of the fluid includes controlling the load (Tr) in a manner not to exceed a rated load of the working medium pump; or alternatively, in that the method further comprises: operating the working medium pump to discharge the working medium at a rated flow rate after the load (Tr) reaches or exceeds the load limit (TrL); and determining whether the detected pressure of the fluid is not less than a threshold pressure (P2T) that is slightly lower than the target pressure of the fluid (P2com), wherein the feedback control of the pressure of the fluid is performed after the pressure of the fluid reaches or exceeds the threshold pressure (P2T). switching a moving direction of the piston when detecting the piston reaching a movement end; detecting a load (Tr) of the working medium pump and determining whether the detected load is not less than a load limit (TrL); operating the working medium pump to discharge the working medium at a maximum flow rate while the load (Tr) is less than the load limit (TrL) after the moving direction of the piston is switched; and detecting a pressure of the fluid and feeding back the detected pressure of the fluid and performing feedback control of the pressure of the fluid to eliminate a difference between the pressure of the fluid and a target pressure of the fluid (P2com), characterized in that the feedback control of the pressure of the fluid is performed after the load (Tr) reaches or exceeds the load limit (TrL), and the feedback control of the pressure of the fluid includes controlling the load (Tr) in a manner not to exceed a rated load of the working medium pump; or alternatively, in that the method further comprises: operating the working medium pump to discharge the working medium at a rated flow rate after the load (Tr) reaches or exceeds the load limit (TrL); and determining whether the detected pressure of the fluid is not less than a threshold pressure (P2T) that is slightly lower than the target pressure of the fluid (P2com), wherein the feedback control of the pressure of the fluid is performed after the pressure of the fluid reaches or exceeds the threshold pressure (P2T). operating the working medium pump to discharge the working medium at a rated flow rate after the load (Tr) reaches or exceeds the load limit (TrL); and determining whether the detected pressure of the fluid is not less than a threshold pressure (P2T) that is slightly lower than the target pressure of the fluid (P2com), wherein the feedback control of the pressure of the fluid is performed after the pressure of the fluid reaches or exceeds the threshold pressure (P2T).