Carbon black filled EPDM (ethylene propylene diene terpolymer) was degraded by hypochlorous acid (HOCl) which is widely used as a bactericide. The strength of the EPDM decreases with the increase in the depth with which HOCl diffused and reacted. Therefore, to evaluate the HOCl resistance of EPDM, it is necessary to clarify the mechanism of degraded layer formation by HOCl. In this study, we aimed to clarify the regularity of degradation layer growth behavior from investigating the time-dependent changes in the degradation depth. As a result, the relation between the degradation depth (D_Cl) and immersion time (t) can be expressed by the power law (D_Cl=Ktⁿ) where K and n are constants, which is known as the corrosion rate equation of a metal material. K was proportional to the concentration of HOCl and was following the Arrhenius law for temperature. n was constant regardless of HOCl concentration and decreased with increasing temperature. We showed that the temporal change of degradation depth can be estimated by the power-law considering concentration and temperature dependence of constants.