A corrosion protection performance degradation detection sensor (7) detects a change in concentration of a corrosion inhibitor, which inhibits corrosion of a corrosion protection object material, added and contained in a solvent. The corrosion protection performance degradation detection sensor (7) includes a detection electrode (1) 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 (2) that is disposed so as to face the detection electrode (1) with a predetermined distance therebetween, and an AC power supply (3) that applies an AC voltage with a predetermined frequency and a predetermined voltage between the detection electrode (1) and the counter electrode (2). 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 (1) at the time when the AC voltage with the predetermined frequency and the predetermined voltage is applied between the detection electrode 1 and the counter electrode (2).