Local sulfidation of steel surfaces was attempted by using a liquid-phase ion gun (LPIG), in which a platinum microelectrode was anodically polarized in the vicinity of the steel surface in 0.1 mol dm^–3 sodium sulfate aqueous solution. An increase in proton concentration, namely local acidification, of the solution in the narrow space between the microelectrode and steel surface successfully induced the formation of an hydrogen sulfide-concentrated environment and resulted in local sulfidation of the steel surface. The degree of sulfidation was dependent on acidification conditions such as the inter-electrode distance, micro-electrode current, specimen electrode potential and composition of steel. The use of an LPIG enables a sulfidation test not only to restrict the amount of hydrogen sulfide generated but also to evaluate sulfidation of a material surface quantitatively.