To provide a manufacturing method of a long length cross-linked polyolefin insulating cable coated with a resin composition excellent in dielectric breakdown characteristics.

The process of manufacturing the composition is characterized in that 0.05 to 0.4 parts by mass of at least one compound chosen from the group consists of bis-(3-ethyl-5-methyl-4-maleimide phenyl) methane, 2,2-bis-[4-(4-maleimide phenoxy) phenyl] propane, N,N'-m-phenylene bis-maleimide, N,N'-4,4'-diphenylmethane bis-maleimide and the compound expressed by formula (1) and a required amount of organic peroxide are added to 100 parts by mass of polyolefin, and the mixture is kneaded to graft the compound onto polyolefin and cooled down to the temperature for extrusion, and after an organic peroxide required for bridging the polyolefin is added, the mixture is extruded onto a conductor to cover it and heated to decompose the peroxide and bridge the polyolefin.