Construction machinery, comprising: a drive source; a fluid pressure pump driven by the drive source; a fluid pressure drive device driven by fluid discharged from the fluid pressure pump; a discharge pressure detection device that detects the discharge pressure from the fluid pressure pump; a fluid pressure motor driven by a return fluid from the fluid pressure drive device; a valve device that controls a flow rate of hydraulic oil discharged from the fluid pressure pump; a first pipeline that allows the return fluid from the fluid pressure drive device to flow out from the valve device to the fluid pressure motor; a generator that is driven by power of the fluid pressure motor and generates electric power; and a motor generator that assists drive of the drive source by electric power supplied from the generator, characterized in that the construction machinery includes: at least one or more flow rate detection device that is provided in the first pipeline and detects the flow rate of the return fluid flowing out to the fluid pressure motor from the fluid pressure drive device; a second pipeline that branches from the first pipeline and allows the return fluid from the fluid pressure device to flow out to a tank; a flow rate control device that is provided in the second pipeline and controls the flow rate of the return fluid from the fluid pressure drive device; and a control device that controls the flow rate control device, and the control device controls the flow rate of the fluid flowing into the fluid pressure motor by controlling the flow rate control device based on the discharge pressure detected by the discharge pressure detection device and the flow rate detected by the flow rate detection device. a drive source; a fluid pressure pump driven by the drive source; a fluid pressure drive device driven by fluid discharged from the fluid pressure pump; a discharge pressure detection device that detects the discharge pressure from the fluid pressure pump; a fluid pressure motor driven by a return fluid from the fluid pressure drive device; a valve device that controls a flow rate of hydraulic oil discharged from the fluid pressure pump; a first pipeline that allows the return fluid from the fluid pressure drive device to flow out from the valve device to the fluid pressure motor; a generator that is driven by power of the fluid pressure motor and generates electric power; and a motor generator that assists drive of the drive source by electric power supplied from the generator, characterized in that the construction machinery includes: at least one or more flow rate detection device that is provided in the first pipeline and detects the flow rate of the return fluid flowing out to the fluid pressure motor from the fluid pressure drive device; a second pipeline that branches from the first pipeline and allows the return fluid from the fluid pressure device to flow out to a tank; a flow rate control device that is provided in the second pipeline and controls the flow rate of the return fluid from the fluid pressure drive device; and a control device that controls the flow rate control device, and the control device controls the flow rate of the fluid flowing into the fluid pressure motor by controlling the flow rate control device based on the discharge pressure detected by the discharge pressure detection device and the flow rate detected by the flow rate detection device. at least one or more flow rate detection device that is provided in the first pipeline and detects the flow rate of the return fluid flowing out to the fluid pressure motor from the fluid pressure drive device; a second pipeline that branches from the first pipeline and allows the return fluid from the fluid pressure device to flow out to a tank; a flow rate control device that is provided in the second pipeline and controls the flow rate of the return fluid from the fluid pressure drive device; and a control device that controls the flow rate control device, and the control device controls the flow rate of the fluid flowing into the fluid pressure motor by controlling the flow rate control device based on the discharge pressure detected by the discharge pressure detection device and the flow rate detected by the flow rate detection device.