The application of sub-terahertz waves is being considered as a method to view the cor­rosion status of internal steel bars. In this study, reflectance intensities were measured for six specimens with different moisture contents using electromagnetic waves from 7.5 to 17.5 GHz. The results showed that the differential reflectance intesity decreased with increasing water con­tent. An equation for estimating the water content at a cover thickness of 10 mm was also calcu­lated. Regarding chloride ion content, no common trend was observed, and it was confirmed that there was a trend in the amount of chloride ions at each cover thickness.