Many nonlinear systems are influenced by undesired external system inputs called disturbances. If such systems cannot be decoupled from the disturbances, it is necessary to compute a control law that reduces the influence of the disturbances with respect to the system outputs. The nonlinear disturbance rejecting controller is based on fundamental linear differential equations for the tracking error and some definitions of differential geometry. This paper presents the new disturbance rejecting controller which is derived and implemented on a testbed (flexible robot). The flexible robot includes as an important subsystem a differential cylinder/servo valve. The robot motion control is realized for the cylinder piston position as the system output. The trajectory tracking shows good performance in terms of a dedicated margin to the disturbance force.