Self-sustained non-linear electrochemical oscillation was investigated with an iron electrode under anodic polarization in an aqueous sulfuric acid solution. The propagation of the reaction front of an oxide formation and reduction along the iron wire electrode was successfully monitored by means of a 16-channel platinum potential probe array. A pulse of the current oscillation was found to consist of both a sequential propagation of the oxide reduction, which makes the rise of the current, and a random formation of the oxide at the electrode surface for the current decay. It was also shown that the traveling velocity of the oxide reduction increased as the potential of the iron electrode became more cathodic. A resistance-inserted in series between an iron electrode and a potentiostat appeared to change the oscillation waveform and the propagation speed of the reduction front. When the propagation was blocked by an insulator coating or a small piece of plastic brick on the electrode, the rising current was disturbed and modulated, having independent occurrence of the reduction propagation at each separated branch of the electrode. These results suggest a coupling between the ohmic potential drop in the solution and the reaction propagation along the electrode surface.