PURPOSE: To eliminate fear of premature cross-linkaging in a cable insulation made of polyethylene and provide it with good mechanical properties and heat resistance by adding an electroconductive carbon black, organic proxide type cross-linkaging agent, and Mg oxide to an ethylene series copolymer resin.

CONSTITUTION: 100 parts by weight ethylene series copolymer resin is mixed with 5-100 parts by wt. electroconductive carbon black, 0.5-5 parts by wt. organic peroxide type cross-linkaging agent, and 0.5-5 parts by wt. Mg oxide with a particle size under 10μm. Accordingly the rising speed of cross-linkaging in an extruding machine is suppressed, and only the degree of cross-linkaging at the final stage can be heightened. That is, the rising speed of cross-linkaging is suppressed. This prevents premature cross-linkaging and provides good mechanical properties and heat resistance.