Negative magnetic permeability of split-ring resonators (SRRs) is theoretically investigated from THz to the visible light region. To describe the frequency dispersion of metal throughout the frequency range, we consider the exact expression of the internal impedance formula. This formula can describe not only the conduction characteristics but also the dielectric behavior of metal in the optical frequency region. Based on these investigations, we successfully determine the magnetic responses of the SRRs from THz to the visible light region. Our results indicate that the behavior of the SRR is changed completely at the transition frequency of 100 THz at which the effect of the reactance in the ring becomes more dominant than that of the resistance in the ring on the SRR.