Reinforced concrete is one of the most widely used materials in social infrastructure due to its economic efficiency, workability, and structural stability. However, chloride attack, which causes deterioration of concrete structures, is a major problem especially in coastal areas and the areas where de-icing agents are sprayed. To solve this problem, various countermeasures have been taken to prevent the penetration of chloride ions into concrete, including salt absorbers, surface coatings and surface-containing agents. In recent years, self-healing concrete has been developed to mitigate steel corrosion by actively lowering the oxygen concentration inside the concrete, thereby inhibiting cathodic reactions. An anaerobic micro-organism (Bacillus subtilis) has also been developed by the authors to inhibit steel corrosion inside concrete. Furthermore, it was found that antioxidants in foodstuffs are effective in inhibiting steel corrosion inside concrete. Green tea has higher antioxidant properties than other substances and is expected to be used as a concrete admixture. In this study, the corrosion inhibition effect of steel bars in mortar mixed with tea leaves was investigated using electrochemical measurements, in particular cathodic polarization curves, which assess the reaction of the cathode of corrosion. The results showed that the cathodic polarization curve of the steel bars in mortar mixed with tea leaves was smaller than that without tea leaves, confirming the potential use of tea leaves as a concrete corrosion inhibitor additive.