Deep learning techniques are increasingly being incorporated into the inspection and maintenance of social infrastructure. In this study, we show that supervised deep learning applied to imaging data obtained sub-THz wave, the average recall exceeded 80% for all cover thicknesses of steel plate inside concrete, and more than 90% for rebar inside concrete with cover thickness up to 20 mm. Unsupervised deep learning enabled the classification for both steel plate and rebar, even at large cover thickness. These results are expected to improve the exploration depth, which has been limited in previous studies.