The vibration control problem of a single pendulum system has been investigated by the vertical movement of a pivot attached to a cart. Since the dynamic equation of the pendulum mounted on a cart is generally a nonlinear time-variant system including the angular acceleration θ^^·(t) in the system parameter, a VSS control scheme having a 4 type switching logic is presented using the Lyapunov's method. By optimally designing the switching plane (s=0) and by linearly approximating the discontinuous function sgs (s) characterizing the VSS near the plane s=0 (i.e., in the optimal range |s|l0.3), satisfactory active damping effects can be obtained both from simulating and practical points of view. Furthermore, in addition to the robust damping effect related to the VSS control, there exists the amplitude-dependent damping that is effective even at larger amplitude.