PURPOSE: To prevent a conductor from changing its color when processed at high temperatures and from corroding under servere use conditions by adding rust preventing agent based on benzotriazole or benzotriazole itself to a watertight compound of base material of ethylene-based copolymer.

CONSTITUTION: Ethylene-vinyl acetate copolymer(WVA), ethylene-ethyl acrylate copolymer(EEA), ethylene-methylmethacrylate copolymer(E-MMA), or ethylene- based copolymer which is metallic acid denatured polymer of these substances has good filling property into the gaps between twisted conductors and adheres well to the conductor and therefore is used as a base material of a watertight compound. A benzotriazole-based rust preventing agent is further added as an additive by 0.5W5wt.%. This material shows excellent conductor protection property even under conditions of high temperature in a cross-linking process of an insulating material and severe use such as hear cycles.