PURPOSE: To stably maintain excellent dielectric breakdown characteristics for a long period by successively forming an inner semiconductive layer, an insulating layer, and an outer semiconductive layer on a conductor, and forming the inner semiconductive layer and the outer conductive layer from a specified cross-linking polyolefin composite.

CONSTITUTION: An inner semiconductive layer, insulating layer, and outer semiconductive layer consisting of resin composites are successively extruded on a conductor to apply a coat thereon, where by a rubber and plastic insulating power cable is manufactured. The inner semiconductive layer and the outer semiconductive layer are formed of a cross linking polyolefin composite containing 100 parts by weights of polyethylene or/and ethylene copolymer, 0.1-20 parts by weights of 1-(2-tert-butylperoxyiospropyl)-3-isopropenylbenzene, and a desired quantity of conductive carbon as essential components. Therefore, electric field relaxation effect is held over a long period, and consequently, excellent dielectric breakdown characteristics can be maintained stably for a long period.