In a lowering operation of a boom, the amount of operation of a control lever is detected by a pressure sensor and input to a controller. Based on the input operation amount, the controller obtains a target flow rate Q0 of return oil discharged from a boom cylinder, calculates a deviation ΔQ between the target flow rate Q0 and an actual flow rate Q obtained from an actual rotation speed N of an electric motor acquired by a rotation speed sensor, calculates a signal Sm for controlling the opening area of a proportional solenoid valve in a manner allowing hydraulic fluid to flow to a control valve in just as much as ΔQ, and controls an operation pilot pressure of the control valve supplied from a sub-pump in accordance with the signal Sm so that the hydraulic fluid will flow to the control valve exactly in the amount of ΔQ.