A semi-active sloshing suppression control method for the contained liquid in the closed vessel is presented. The presented method uses the air spring effect of the gas chamber on the liquid surface, when the closed vessel is divided into two sectors with the vertical bulkhead in it so as to allows communication between two sectors at the lower part of the vessel. When the valve that is provided at the bulkhead between two gas chambers closes, the spring effect of the gas chamber arises. On the contrary, when the valve opens, the spring effect of the gas chamber diminishes. Switching algorithm of the valve operation makes sloshing response to decrease. The effectiveness of the presented semi-active sloshing suppression control method is examined with numerical simulations and the scaled model experiment. As the result, the presented method is effective to suppress the sloshing while the consuming energy to control the valve is small.