PURPOSE: To find corrosion reaction resistance used to evaluate a degradation degree in a short time in an AC impedance method to detect a degraded situation of a metallic member on which self-restorable surface treatment such as chromate treatment is performed.

CONSTITUTION: In a degradation degree detecting method performed by measuring an impedance by impressing an alternating current during the time when a pair of samples are soaked in measuring solution by using a self-restorable surface treated metallic member as a sample of a degradation degree detecting object, an AC frequency used in an AC impedance method is selected as a frequency where corrosion reaction resistance Rc is unrecognizably contained in a substantial constitutive element of an impedance Z to be measured. The corrosion reaction resistance Rc is calculated by multiplying the impedance Z measured by an alternating current having the frequency by a predetermined proportional constant N.