The Luenberger observer is frequently used for the state estimator of linear time-invariant control systems. However, it cannot be applied directly to the plant which has varying parameters. This paper provides an observer for this problem whose structure is not affected by the varying parameters of the plant. The results of this paper give the existence condition and the stability condition of such an observer and provide a direct design procedure. The extension of this idea to the design of an unknown input observer is also presented