With developing technology, automobile has progressed a comfortable one and chassis control system related to this is studied vigorously.Suspension plays an important role as vibration suppression and handling. But the conventional passive suspension cannot improve those two simultaneously. Therefore it necessitates a demand on the several kinds of electronic-controlled suspensions.In this paper the design of sliding mode controller using singular perturbation method for 1/4 car semi-active suspension model is studied. Using singular perturbation method, the full order system is separated into slow subsystem related to sprung mass and fast subsystem related to unsprung mass. With this model reduction, sliding mode controller which is based on Lyapunoy theory is designed and applied to real system easily. Sliding mode control which is designed to be robust with changing system parameters is applied to slow subsystem. To verify the effectiveness of the proposed algorithm several simulations and experiments have been conducted and to show robustness of proposed algorithm simulations and experiments have been conducted with changing sprung mass.With results of simulation and experiments, the effectiveness of proposed algorithm is confirmed and the robustness of proposed algorithm is confirmed also.