An electro-hydraulic drive system for a work machine which drives an upper rotary body by controlling a flow rate of a hydraulic fluid by a control valve, includes an electro-hydraulic pump including a hydraulic pump driven by an engine, and a first electric motor, and an electro-hydraulic motor including a hydraulic motor rotated by the hydraulic oil supplied from the electro-hydraulic pump, and a second electric motor, and a controller which determines an operating state of the first electric motor and an operating state of the second electric motor based on a load of the electro-hydraulic pump and a load of the electro-hydraulic motor, and is able to perform an efficient operation by using a hydraulic pressure (fluid pressure) and electric power.