To elucidate the mechanism of corrosion inhibition of mild steel by polyphenol compounds (gallic acid, tannic acid, etc.), the action of gallic acid (GA) in neutral solution was investigated by physicochemical methods. While GA had a corrosion inhibiting efficiency of about 99% in air, it had no such action in an oxygen-free atmosphere. Electrochemical measurements showed that GA was a cathodic inhibitor and various other methods demonstrated that it was an absorption (type II) inhibitor rather than an oxygen scavenger (type I) inhibitor. As for the mechanism of GA corrosion inhibition, it is thought that i) GA molecules are electrostatically attracted to the steel surface by the φ-potential theory, ii) GA molecules adsorb physically and chemically on the surface, and iii) an adsorption layer like a membrane forms on the surface to inhibit the corrosion of steel.