PID(Proportional-Integral-Derivative) controller structure is the most widely used one in industrial applications. Yamada and Hagiwara proposed a design method for modified PID controllers such that modified PID controllers make the control system for unstable plants stable and the admissible sets of P-parameter, I-parameter and D-parameter are independent from each other. When modified PID control systems are applied to real plants, the influence of disturbance in the plant must be considered. In many cases, disturbance in the plant is unknown. It is comparatively easy to attenuate known disturbance, but it is difficult to attenuate unknown disturbances. From a practical viewpoint, it is desirable to design a modified PID control system to attenuate unknown disturbances. However, no paper examines a design method for modified PID control systems to attenuate unknown disturbances. In this paper, we propose a design method for modified PID control systems to attenuate unknown disturbances.