The ac impedance method has been applied to evaluate the degradation of an organic-coated stainless steel with a macroscopic line defect. The delamination propagated in the direction normal to the macroscopic line defect during the corrosion test, and a thin electrolyte layer was formed under the delaminated coating. The EIS data are interpreted in terms of a model in which the equivalent circuit of stainless steel/coatings/solution interfaces is composed of a parallel combination of the circuits for a macroscopic defective part and for a delaminated part. The former can be expressed by a parallel circuit of a polarization resistance and a double layer capacitance, while the latter can be explained by a one-dimensional distributed constant circuit. The impedance parameters in the proposed equivalent circuit were determined by curve fitting of the EIS data. The interfacial capacitance correlated well with the actual delaminated area. The EIS technique is very promising for the evaluation of organic coating with a macroscopic defect.