By making clear the relationship between the cathodic limiting current density and R-type Warburg impedance used in the Randles type equivalent circuit, the Limiting current density, as well as the exchange current density, of a porous La0.6Sr0.4Mn3+delta/YSZ electrode has been successfully estimated from a complex impedance spectrum measured at an equilibrium potential, ii steady-state polarization calculated from these current densities gave a good agreement with the measured one, which indicates that, even at an equilibrium potential, the limiting current density can be obtained with high precision from the complex impedance. Investigation of the exchange and limiting current densities of the electrode under an oxygen partial pressure, P-O2, of 0.001-1 atm at 1123-1273 K has derived the formulation for expressing an equation which expresses the P-O2 and temperature dependence of steady-state polarization.