A gas fuel supply device for supplying liquefied gas at a high pressure for injection of fuel gas into a combustion chamber of a high-pressure gas injection diesel engine, the gas fuel supply device comprising: a plurality of gas fuel supply systems each comprising: a reciprocating pump arranged to be driven by a hydraulic motor to pressurize introduced liquefied gas to a desired pressure, and discharge the pressurized liquefied gas; a hydraulic system including a hydraulic pump unit arranged to supply driving hydraulic pressure from an electric motor-driven variable-capacity hydraulic pump to the hydraulic motor, and a vacuum prevention line which branches off from a hydraulic pressure supply line connecting the hydraulic pump unit and the hydraulic motor, and which is connected to an auxiliary oil reservoir tank, wherein the vacuum prevention line is provided with a check valve for blocking a hydraulic pressure flow in a direction from the hydraulic pressure supply line toward the auxiliary oil reservoir tank; and a heating device arranged to heat and vaporize the pressurized liquefied gas supplied from the reciprocating pump; a control part arranged to regulate rotational speed of the hydraulic motor using the gas fuel outlet pressure of the heating device or the rotational speed of the hydraulic motor as a controlled object; an engine inlet gas pressure-reducing valve arranged to regulate pressure of gas fuel to be injected into the combustion chamber; and a recirculation line including a flow regulating valve arranged to branch off the liquefied gas, which has been pressurized by the reciprocating pump, from an upstream side of the heating device into a suction drum, wherein the control part is arranged to, in a low speed region where the pump load would otherwise be small, increase the opening degree of the flow regulating valve to increase the recirculation flow through the recirculation line to secure a total flow of the liquefied gas flow through the reciprocating pump to maintain the rotational frequency of the hydraulic motor within a rotational frequency region where the hydraulic motor can be controlled, and wherein hydraulic systems of the respective gas fuel supply systems are connected with each other in an interchangeable manner. a plurality of gas fuel supply systems each comprising: a reciprocating pump arranged to be driven by a hydraulic motor to pressurize introduced liquefied gas to a desired pressure, and discharge the pressurized liquefied gas; a hydraulic system including a hydraulic pump unit arranged to supply driving hydraulic pressure from an electric motor-driven variable-capacity hydraulic pump to the hydraulic motor, and a vacuum prevention line which branches off from a hydraulic pressure supply line connecting the hydraulic pump unit and the hydraulic motor, and which is connected to an auxiliary oil reservoir tank, wherein the vacuum prevention line is provided with a check valve for blocking a hydraulic pressure flow in a direction from the hydraulic pressure supply line toward the auxiliary oil reservoir tank; and a heating device arranged to heat and vaporize the pressurized liquefied gas supplied from the reciprocating pump; a reciprocating pump arranged to be driven by a hydraulic motor to pressurize introduced liquefied gas to a desired pressure, and discharge the pressurized liquefied gas; a hydraulic system including a hydraulic pump unit arranged to supply driving hydraulic pressure from an electric motor-driven variable-capacity hydraulic pump to the hydraulic motor, and a vacuum prevention line which branches off from a hydraulic pressure supply line connecting the hydraulic pump unit and the hydraulic motor, and which is connected to an auxiliary oil reservoir tank, wherein the vacuum prevention line is provided with a check valve for blocking a hydraulic pressure flow in a direction from the hydraulic pressure supply line toward the auxiliary oil reservoir tank; and a heating device arranged to heat and vaporize the pressurized liquefied gas supplied from the reciprocating pump; a control part arranged to regulate rotational speed of the hydraulic motor using the gas fuel outlet pressure of the heating device or the rotational speed of the hydraulic motor as a controlled object; an engine inlet gas pressure-reducing valve arranged to regulate pressure of gas fuel to be injected into the combustion chamber; and a recirculation line including a flow regulating valve arranged to branch off the liquefied gas, which has been pressurized by the reciprocating pump, from an upstream side of the heating device into a suction drum, wherein the control part is arranged to, in a low speed region where the pump load would otherwise be small, increase the opening degree of the flow regulating valve to increase the recirculation flow through the recirculation line to secure a total flow of the liquefied gas flow through the reciprocating pump to maintain the rotational frequency of the hydraulic motor within a rotational frequency region where the hydraulic motor can be controlled, and wherein hydraulic systems of the respective gas fuel supply systems are connected with each other in an interchangeable manner.