A feasibility study was conducted on the detection of corroded rebars inside concrete using sub-THz waves. In this study, spectral measurement were performed with a reflection type system in the 20-50 GHz frequency range. Measurements were conducted on steel plates corroded in salt water and on concrete specimens containing these corroded plates. The results confirmed that reflectance decrease as corrosion progress. Furthermore, it was demonstrated that the presence or absence of internal steel plates and corrosion can be detected up to a cover thickness of 20 mm. The frequency spectral peaks and their periodicity also provided a means to estimate the cover thickness.