Surface plasmon excitations and the associated optical transmission properties in perforated metal/dielectric/metal trilayer structures are numerically investigated. Pronounced magnetic modes are observed in the antisymmetric and asymmetric modes of surface plasmon polaritons (SPPs). The influence of substrates on the magnetic response is studied in detail. Quite different from the conventional LC-circuit resonance, these magnetic excitations arise from the nonlocalized SPPs in the perforated layered structure, which may considerably enrich the electromagnetic properties of such metamaterials, especially the artificial magnetism at optical frequency.