A liquid crystal (LC) lens can make any lens state from positive (convex) to negative (concave) by applying an external voltage. However, when the LC lens changes its focal length, it is difficult to say its response time is fast enough. In this paper, the authors aim at making the response time of the LC lens be shorter by using a control theory to a value of the external voltage applied to the LC lens. We constructed an automatic control system which can change the focal length faster by a computer program. The computer program gives appropriate effective values of AC voltages with the LC lens based on a feed forward control. We conducted experiments to change the focal length of the LC lens from a non-lens state to 200mm and-100mm, respectively. When our proposed effective voltage control method was performed, the response time was successfully reduced in half, compared to a case when a simple step-like voltage was applied.