A power regeneration device for an operating machine equipped with an engine, a hydraulic pump driven by the engine, a control valve for switching the flow of hydraulic fluid from the hydraulic pump and supplying the hydraulic fluid to a hydraulic cylinder, and an operating device for controlling the control valve, comprising: a hydraulic line which is connected to a bottom-side hydraulic chamber of the hydraulic cylinder and through which returning fluid returning to a tank flows when the hydraulic cylinder contracts; a branching part which is provided in the hydraulic line to separate the hydraulic line into a plurality of hydraulic lines; a regeneration line which is connected to the branching part and leads the returning fluid to the tank via a hydraulic motor that is connected with a generator controlled by an inverter; a control valve line which is connected to the branching part and leads the returning fluid to the tank via the control valve; operation amount detection means which detects the operation amount of the operating device; an electricity storage device which stores electric power generated by the generator; electric amount detection means which detects the electric amount of the electricity storage device; characterized in further comprising target flow rate calculation means which calculates a target flow rate of the returning fluid flowing through the regeneration line and a target flow rate of the returning fluid flowing through the control valve line depending on an electric amount signal from the electric amount detection means; first flow rate control means which controls the flow rate through the control valve line based on the result of the calculation by the target flow rate calculation means; and second flow rate control means which controls the flow rate through the regeneration line based on the result of the calculation by the target flow rate calculation means. a hydraulic line which is connected to a bottom-side hydraulic chamber of the hydraulic cylinder and through which returning fluid returning to a tank flows when the hydraulic cylinder contracts; a branching part which is provided in the hydraulic line to separate the hydraulic line into a plurality of hydraulic lines; a regeneration line which is connected to the branching part and leads the returning fluid to the tank via a hydraulic motor that is connected with a generator controlled by an inverter; a control valve line which is connected to the branching part and leads the returning fluid to the tank via the control valve; operation amount detection means which detects the operation amount of the operating device; an electricity storage device which stores electric power generated by the generator; electric amount detection means which detects the electric amount of the electricity storage device; characterized in further comprising target flow rate calculation means which calculates a target flow rate of the returning fluid flowing through the regeneration line and a target flow rate of the returning fluid flowing through the control valve line depending on an electric amount signal from the electric amount detection means; first flow rate control means which controls the flow rate through the control valve line based on the result of the calculation by the target flow rate calculation means; and second flow rate control means which controls the flow rate through the regeneration line based on the result of the calculation by the target flow rate calculation means.