A hydraulic fluid energy regeneration device for a work machine, comprising: - a first hydraulic actuator; - a regeneration hydraulic motor (13; 62) driven by return hydraulic fluid discharged by the first hydraulic actuator; - a first hydraulic pump mechanically connected to the regeneration hydraulic motor (13; 62); - a second hydraulic pump that delivers the hydraulic fluid for driving the first hydraulic actuator and/or a second hydraulic actuator (3b, 3c, 3d); - a junction line that allows the hydraulic fluid delivered by the first hydraulic pump to join the hydraulic fluid delivered by the second hydraulic pump; - a first regulator (15A; 17) that regulates the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line; - a second regulator that regulates the delivery flow rate of the second hydraulic pump; and - a control unit to which a target displacement command for the second hydraulic pump is input, the control unit calculating the flow rate of the hydraulic fluid delivered by the first hydraulic pump and the flow rate of the hydraulic fluid delivered by the second hydraulic pump in accordance with the target displacement command, the control unit further outputting a control command to the first regulator (15A; 17) and a control command to the second regulator in accordance with the calculated flow rates, characterized in that: the control unit includes: - a first computing part (103, 109, 110, 111; 103, 109, 122, 123, 124, 125) that calculates a demanded pump flow rate in accordance with the input target displacement command for the second hydraulic pump, the first computing part (103, 109, 110, 111; 103, 109, 122, 123, 124, 125) further outputting a control command to the first regulator (15A; 17) in such a manner that the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line is equal to or lower than the demanded pump flow rate; and - a second computing part (112, 113) that subtracts from the demanded pump flow rate the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line to obtain a target pump flow rate, the second computing part (112, 113) further outputting a control command to the second regulator in such a manner that the calculated target pump flow rate is attained. - a first hydraulic actuator; - a regeneration hydraulic motor (13; 62) driven by return hydraulic fluid discharged by the first hydraulic actuator; - a first hydraulic pump mechanically connected to the regeneration hydraulic motor (13; 62); - a second hydraulic pump that delivers the hydraulic fluid for driving the first hydraulic actuator and/or a second hydraulic actuator (3b, 3c, 3d); - a junction line that allows the hydraulic fluid delivered by the first hydraulic pump to join the hydraulic fluid delivered by the second hydraulic pump; - a first regulator (15A; 17) that regulates the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line; - a second regulator that regulates the delivery flow rate of the second hydraulic pump; and - a control unit to which a target displacement command for the second hydraulic pump is input, the control unit calculating the flow rate of the hydraulic fluid delivered by the first hydraulic pump and the flow rate of the hydraulic fluid delivered by the second hydraulic pump in accordance with the target displacement command, the control unit further outputting a control command to the first regulator (15A; 17) and a control command to the second regulator in accordance with the calculated flow rates, characterized in that: the control unit includes: - a first computing part (103, 109, 110, 111; 103, 109, 122, 123, 124, 125) that calculates a demanded pump flow rate in accordance with the input target displacement command for the second hydraulic pump, the first computing part (103, 109, 110, 111; 103, 109, 122, 123, 124, 125) further outputting a control command to the first regulator (15A; 17) in such a manner that the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line is equal to or lower than the demanded pump flow rate; and - a second computing part (112, 113) that subtracts from the demanded pump flow rate the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line to obtain a target pump flow rate, the second computing part (112, 113) further outputting a control command to the second regulator in such a manner that the calculated target pump flow rate is attained. - a first computing part (103, 109, 110, 111; 103, 109, 122, 123, 124, 125) that calculates a demanded pump flow rate in accordance with the input target displacement command for the second hydraulic pump, the first computing part (103, 109, 110, 111; 103, 109, 122, 123, 124, 125) further outputting a control command to the first regulator (15A; 17) in such a manner that the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line is equal to or lower than the demanded pump flow rate; and - a second computing part (112, 113) that subtracts from the demanded pump flow rate the flow rate of the hydraulic fluid coming from the first hydraulic pump and flowing through the junction line to obtain a target pump flow rate, the second computing part (112, 113) further outputting a control command to the second regulator in such a manner that the calculated target pump flow rate is attained.