The authors examined the possibility of terahertz (THz) absorption spectroscopy whether it can be used as a tool to estimate the content of talc in ethylene-propylene-diene copolymer (EPDM), an important factor determining the electrical insulation properties of EPDM-insulated power cables. For this purpose, absorption spectra were measured in a frequency range from 0.3 to 10.0 THz for EPDM sheets containing different contents of talc, a disk-shaped sample made of talc powder, and a similar disk-shaped sample made of a mixture of talc and high-density polyethylene. As a result, the whole absorption in the frequency range from 0.3 to 10.0 THz is increased by the addition of talc into EPDM sheets. More specifically, it has become clear that talc in EPDM induces absorption peaks at 3.05, 4.8, 5.1, 5.3, 6.9, and 7.8 THz. The integrated absorption coefficients calculated for these absorption peaks show a good linearity with respect to the content of talc. Therefore, by watching the increments of these absorption peaks, the content of talc can be quantitatively estimated.