Disclosed is a method for estimating, within a short time, the life of a cable insulating coating material containing an antioxidant in a suitable concentration based on the rate of decrease of the antioxidant and on the critical concentration of the antioxidant at which oxidative degradation rapidly proceeds. The method tests the coating material to examine its life, the coating material including a base polymer, and an antioxidant having a functional group suppressing an oxidative deteriorative reaction of the base polymer. The method includes performing a thermal degradation test on the coating material; determining the degradation levels and degradation rates of the coating material at two or more time points in the thermal degradation test, based on the ratio of the absorbance of the functional group of the antioxidant to the absorbance of the base polymer; and thereby evaluating the life of the coating material.