To understand details of the corrosion protection mechanism of steel in a commercially available aqueous solution of antifreeze agent containing a corrosion inhibitor, immersion tests were carried out for 168 hours in aqueous solutions prepared by adding sodium pyrophosphate, sodium gluconate or sodium citrate to rock salt. In the weight change of the steel specimens after the immersion test, the weight loss of the steel was smaller in all the aqueous solutions than in the rock salt only aqueous solution. From the SEM observations of the steel surface after immersion of the solution containing sodium pyrophosphate, a several micron thick film was formed in the inhibited corrosion part, and O, Fe, P and small amounts of Mg and Ca were detected by the EDS analysis. In the FT-IR measurements of the steel surface, an absorption peak corresponding to the PO43－ structure was measured. The structure of this was different from the surface film formed by the NaCl aqueous solution containing inhibitor in previous reports.