A Series Elastic Actuator (SEA) is designed by placing a passive compliant element between a conventional stiff actuator and link. The intrinsically compliant mechanical structure provides several superiorities, e.g., safety, energy efficiency, high force fidelity, low cost force measurement, high transparency, etc., in advanced robot applications, such as humanoids, quadrupeds and exoskeletons. However, the motion control problem of an SEA is more complicated than that of a conventional stiff actuator due to its higher order dynamics. This paper proposes a novel Active Disturbance Rejection (ADR) based robust force controller for SEAs by combining Differential Flatness (DF) and Disturbance Observer (DOb) in state space. The robust state and control input references are systematically generated in terms of a fictitious variable, namely differentially flat output, estimated disturbances and their successive derivatives. A second order DOb is designed in state space so that disturbances and their first and second order derivatives are estimated. It is experimentally shown that high performance force control applications can be performed without requiring the precise dynamic models of the actuator and environment when the proposed robust force controller is implemented.