Multiloop control systems are often used for industrial multivariable processes because of their simplicity. To design multiloop control systems, single-input single-output (SISO) methods that guarantee specified closed-loop characteristics can be applied. Because the design equations are nonlinear due to loop interactions and may cause computational difficulties, a continuation method is proposed to obtain the solutions. By choosing the interaction level as a continuation parameter, we can design multiloop control systems without worrying about numerical difficulties such as divergence. To illustrate the approach, the modified Ziegler-Nichols method is applied to design multiloop control systems.