A working machine equipped with a hydraulic actuator (3, 114, 115) for driving a work device, a control valve for operating and controlling the hydraulic actuator (3, 114, 115), control lever device with a control lever for operating the control valve to activate the hydraulic actuator (3, 114, 115) and a power regeneration device, the power regeneration device comprising: a hydraulic motor driven by return oil from the hydraulic actuator (3, 114, 115); an electric motor connected mechanically to the hydraulic motor and driven thereby to generate electric power; an inverter which controls the rotation speed of the electric motor, and an electric storage device which stores the electric power generated by the electric motor; wherein the return oil discharged from the hydraulic actuator (3, 114, 115) is branched and distributed to the side of the control valve and that of the hydraulic motor, the power regeneration device further comprising: a rotation speed detector which detects an actual rotation speed of the electric motor; an operation amount detector which detects the amount of operation of the control lever; a proportional solenoid valve which adjusts the opening area of the control valve, and a control device to which the rotation speed detected by the rotation speed detector and the operation amount detected by the operation amount detector are input; characterized in that, the control device obtains a target flow rate of the return oil discharged from the hydraulic actuator (3, 114, 115) and a target rotation speed of the electric motor based on the operation amount to control the rotation speed of the electric motor via the inverter in a manner attaining the target rotation speed of the electric motor, and obtains a deviation between the target flow rate Qo of the return oil discharged from the bottom-side hydraulic chamber of the hydraulic actuator and an actual flow rate of hydraulic fluid passing through the hydraulic motor based on the actual rotation speed of the electric motor detected by the rotation speed detector, and controls the proportional solenoid valve based on the deviation obtained. a hydraulic motor driven by return oil from the hydraulic actuator (3, 114, 115); an electric motor connected mechanically to the hydraulic motor and driven thereby to generate electric power; an inverter which controls the rotation speed of the electric motor, and an electric storage device which stores the electric power generated by the electric motor; wherein the return oil discharged from the hydraulic actuator (3, 114, 115) is branched and distributed to the side of the control valve and that of the hydraulic motor, the power regeneration device further comprising: a rotation speed detector which detects an actual rotation speed of the electric motor; an operation amount detector which detects the amount of operation of the control lever; a proportional solenoid valve which adjusts the opening area of the control valve, and a control device to which the rotation speed detected by the rotation speed detector and the operation amount detected by the operation amount detector are input; characterized in that, the control device obtains a target flow rate of the return oil discharged from the hydraulic actuator (3, 114, 115) and a target rotation speed of the electric motor based on the operation amount to control the rotation speed of the electric motor via the inverter in a manner attaining the target rotation speed of the electric motor, and obtains a deviation between the target flow rate Qo of the return oil discharged from the bottom-side hydraulic chamber of the hydraulic actuator and an actual flow rate of hydraulic fluid passing through the hydraulic motor based on the actual rotation speed of the electric motor detected by the rotation speed detector, and controls the proportional solenoid valve based on the deviation obtained. a rotation speed detector which detects an actual rotation speed of the electric motor; an operation amount detector which detects the amount of operation of the control lever; a proportional solenoid valve which adjusts the opening area of the control valve, and a control device to which the rotation speed detected by the rotation speed detector and the operation amount detected by the operation amount detector are input; characterized in that, the control device obtains a target flow rate of the return oil discharged from the hydraulic actuator (3, 114, 115) and a target rotation speed of the electric motor based on the operation amount to control the rotation speed of the electric motor via the inverter in a manner attaining the target rotation speed of the electric motor, and obtains a deviation between the target flow rate Qo of the return oil discharged from the bottom-side hydraulic chamber of the hydraulic actuator and an actual flow rate of hydraulic fluid passing through the hydraulic motor based on the actual rotation speed of the electric motor detected by the rotation speed detector, and controls the proportional solenoid valve based on the deviation obtained.