Rebar corrosion and corrosion-induced crack are one of the major deteriorations of concrete structures. The corrosion-induced crack accelerates the corrosion and deterioration processes and can involve concrete cover delamination. It is necessary to assess rebar corrosion and internal damage to avoid cover delamination and to judge the damage level during maintenance procedures. The authors have been developing a non-destructive test method to detect both of rebar corrosion and internal crack area by using electromagnetic wave radar. The feature of developed assessment method is that the Self-Organizing Map (SOM) is applied to evaluate the change of reflection waveform data obtained by electromagnetic wave radar due to rebar corrosion and internal crack. The specimens with a cover thickness of 30 and 60 mm with corrosion-induced internal crack were made for the electrolytic corrosion experiment and the developed assessment method was applied. As a result, in the case of the specimen before corrosion, it was distinctly classified as rebar and concrete. The result for the specimens after corrosion showed that the rebar corrosion area was different from the sound case. Moreover, the area well coincided with the internal crack area. The developed method succeeded to detect quantitatively not only rebar corrosion position but also the internal crack area of more than 0.1mm crack width for the condition of cover thickness less than 60 mm and corrosion ratio larger than about 4%.