For semiactive vibration suppression of truss structures with variable dampers, on-off control laws are newly derived suggested by the sliding-mode control theory. The control laws are shown to be in the same form as previously proposed control laws which are derived suggested by LQR control theory. The vibration suppression performances of the new control laws are investigated and compared with those of the previously proposed control laws by using numerical simulations. The results of the simulation show that the vibration suppression by the new control laws is effective and that performances of them are approximately the same as the previous laws although they result in different values of control law constants.