A silver/silver chloride microelectrode was developed as a liquid-phase ion gun for investigation of precursor processes of pitting corrosion. This electrode was set as a probe electrode of a scanning electrochemical microscope and used to induce a local breakdown of passive film on iron. A small amount of chloride ions generated by cathodic polarization of the silver/silver chloride microelectrode has succeeded in inducing the local breakdown of the passive film formed on iron in deaerated pH 6.5 berate solution. Moreover, the microelectrode could detect ferric ions dissolved from the film at the initial stage of the film breakdown.