The LLC converter is a very effective topology for miniaturization of power supplies. However, its application is currently limited. That is because of the problem of degraded system stability when the load condition changes. This paper proposed a control system that controls the output voltage by adjusting the power factor cosθ of the LLC converter to address this problem. As a result, simulations confirm that the proposed control method can significantly improve the frequency characteristics of the LLC converter and shows stable control characteristics regardless of the operating frequency. We also designed an experimental control circuit that can realize the proposed control.