A corrosion protection performance degradation detection sensor includes a detection electrode that reacts with the corrosion inhibitor dissolved in the solvent and forms on a surface thereof an electrode surface film which inhibits corrosion, a counter electrode that is disposed so as to face the detection electrode, and an AC power supply that applies an AC voltage between the detection electrode and the counter electrode. A change in concentration of the corrosion inhibitor in the solvent is detected, on the basis of a change in impedance of the electrode surface film formed on the surface of the detection electrode.