A method is presented to suppress an unbalanced vibration of rotors supported at both ends. In this method, only one end of the rotor is supported by a spring and a damper, and the bearing box mass m, the viscous damping coefficient c and the spring constant k in the support are determined in order to suppress the rotor amplitude by applying the theory of dynamic absorbers. In this case, the rotor supported by the spring-damper needs to be replaced by the equivalent two-degree-of-freedom system. Two approaches for the replacement, that is, an analytical and an experimental approach, are developed. It is found from the experiment and the numerical simulation that the spring-damper support with m, c and k obtained from both approaches suppresses the rotor vibration very well. The characteristics of both approaches are shown.