Timely input sometimes can control periodic movement of mechanical systems. In the swing which is the parametric excitation system, when the center of gravity is moved in double cycle, the swing's amplitude increases, and it decreases when this phase is moved π [rad] . But it is difficult to control a swing in the wide range, because the cycle of the swing depends on its amplitude. This paper shows that the van der Pol's equation inputted the swing's angular velocity can compose the frequency observer, and proposed the new control form of a swing that the center of gravity is moved by nonlinear feedback which consists of equation's output.