Electromagnetic waves are named depending on their wavelength range. For many years, the generation methods, detection methods and applications of almost all electromagnetic waves have been investigated. However, there remains an unveiled range owing to a lack of appropriate generation methods. This wavelength range is from several tens to several hundreds of micrometers, and has a frequency range from one to several tens of THz. Generally, waves in this range are called the THz waves. Some generation methods for THz waves have been developed recently, the characteristics of these waves are still under investigation, but some applications have been proposed. One of the important properties of THz waves is their high transmissivity in opaque dielectric materials. Generally, insulating materials are opaque to visible light, and it is difficult to observe internal defects externally. However, if THz waves show sufficient transmissivity and proper interaction with internal physical quantities, they are expected to be a useful measurement tool for insulating materials. In this study, polyethylene, which is a typical insulating material, is choosen as the measurement target, and void interfaces and mechanical strain are detected using THz waves.