In this paper, we examine a design method for unknown input observers for non-minimum phase systems. The unknown
input observer has been used to estimate the state variable of the plant in the presence of unknown input. In addition the
unknown input observer is applied to the systems with disturbance inputs or unknown varying parameters. Initially, the
unknown input observer is examined by Kudva, Viswanadham and Ramakrishna. According to past studies, the unknown
input observer for the plant (A,B,C,0) can be designed if and only if following 2 expressions hold true: (1) rankCB= rankB and (2) the plant (A,B,C,0) has no invariant zero in the closed right half plane. Many papers have
been considered to design unknown input observers, even if above mentioned 2 expressions are not satisfied. In this
paper, we propose a new design method of unknown input observers for non-minimum phase plants.