PURPOSE: To obtain the subject insulated material excellent in oxidative deterioration resistance characteristics under high-temperature and high electric field, suppressed in increase in tan δ and useful as an electric cable.

CONSTITUTION: This insulated material contains (A) 100 pts.wt. of a polyolefin, 0.1 pts.wt. of a mixture of (B) 10-50wt.% of a compound of formula I (R is C12H25 or C14H29) and (C) 50-90wt.% of a compound of formula II (R1 and R2 are each C12H25 or C14H29) and (D) an organic peroxide. Furthermore, e.g. benzoyl peroxide is preferably used as the component D.