In this paper, a simultaneous design method for systems with variable inertial term is proposed. Because conventional control methods can hardly deal with the system due to the variable inertial term, a noninteracting control is applied to linearize the system. In addition, to insure the reliability of the imperfectly linearized system, evaluation methods in H_2 and H_∝ are determined. Through the linearization method and the evaluation methods, conventional linear control methods can be applied to the system. The property of this structure system concerns the control performance so that the simultaneous design method of structure and control is employed to design the controlled system. This design method is applied to an example of a system with variable inertial term, and it is confirmed that the method can design the controller and improve the control performance.