This paper presents a nonlinear variable stiffness rotary joint using antagonistic drives, series elastic actuators, and polymer elastic materials. The proposed mechanism is composed of two pairs of opposed Neidhart rubbers and series elastic mechanisms, that make the stiffness harder by compressing the elastic materials. From the experiments, it was revealed that the nonlinear variable joint stiffness was achieved by the antagonism and the redundant drive angle control worked although the joint viscosity did not vary drastically.