In planer-type optical devices such as OLED, the loss-propagating surface plasmon polaritons (SPPs) at the interface on a metallic cathode has been regarded as holding the key to the efficiency improvement. It complicates the characteristics of SPPs, however, that the existence of the classical or phenomenological surface electromagnetic (EM) propagation modes at the interface. With the aid of tracing the predecessors' prior achievements, we clarify theoretically the characteristic of surface EM modes over a comprehensive variety of a permittivity of the interfacing medium. As a result, we prove that there is a requisite in order that the surface EM excitations might be quasi independently observed in a vertical dipole radiation nearby the interface, which is non-positive real part of a permittivity of the medium interfacing to the air, such as a real metal, involving the condition for existence of SPPs.