Many of Japan’s infrastructures are more than 50 years old, and more old infrastructures are expected in the future. Therefore, the challenge is to inspect the deteriorated condition of those infrastructures. This study focuses on the use of electromagnetic waves in NDT as an inspection method for infrastructure. One issue with this method is that the device is not suitable for actual infrastructure inspections. We constructed a portable reflection measurement system that utilizes sub-terahertz waves among electromagnetic waves. Reflection measurements were conducted on corroded and uncorroded steel plates using this compact system. It may be possible to identify samples using peak shift with a horn antenna. With a probe antenna, identification is based on reflectance.