Inorganic fillers are very often added to ethylene-propylene-diene copolymer (EPDM), which is a typical rubber material used for electrical insulation, for the purpose of improving its various properties. Regarding this, although the content and dispersion of inorganic fillers exert a significant influence on the properties of EPDM, detailed research has been done scarcely. In the present research, talc and magnesium hydroxide were added to EPDM, and detection of the content and dispersion of fillers was tried by terahertz absorption spectroscopy. As a result, it has become clear that the content of talc can be estimated by the increase in absorption at 3.05 THz. The possibility of estimating the talc content from incremental values of three absorptions in a far-infrared range has also been confirmed, in addition to the 3.05-THz absorption. Furthermore, the dispersion of magnesium hydroxide fillers is likely to correspond to the scattering of absorption spectra at around 1.0 to 1.5 THz.