To provide a polyphenol sensor with a longer life thereof which permits simple and quick measurement of polyphenol.

The polyphenol sensor employs an enzyme electrode containing enzyme for decomposing hydrogen peroxide and an electronic mediator, adds the enzyme for decomposing the hydrogen peroxide and a reagent into a reaction tank containing a fixed concentration of hydrogen peroxide and measures the amount of polyphenol in a sample liquid by electrochemically measuring decrease in the concentration of hydrogen peroxide as shown a fixed time later. Sodium azide is contained in the reaction tank.