Non-minimum phase plants cause fundamental design constraints, such as bandwidth limitation, in the feed-back control systems due to their peculiar phase characteristics. Therefore, the control problem of non-minimum phase systems is quantifiably harder than the minimum phase's one. Furthermore, the control problem becomes more severe in the presence of system uncertainties and external disturbances. In this paper, we propose a disturbance observer (DOB) based solution for the robust control problem of non-minimum phase systems. This paper shows that non-minimum phase systems limit the DOB's bandwidth, which is one of the most significant design parameters in the DOB based robust control systems, to satisfy robustness. The Poisson integral formula is utilized so that the bandwidth limitations are derived analytically. As a result, a new analysis and design tool is proposed for the robust control problem of non-minimum phase systems by using the DOB. The proposed method is implemented into two different non-minimum phase plant models, which have right half plane (RHP) zero and pole, and its validity is shown by given simulation results.