A resin composition includes a silane-crosslinked polyethylene as a main component provided by crosslinking a polyethylene in the presence of a silanol condensation catalyst and water, the polyethylene being graft-copolymerized with a silane compound by a free radical generating agent. The polyethylene has a density of 0.87 to 0.91 g/cm3. The silane compound is included not less than 2 parts by mass relative to 100 parts by mass of the polyethylene. The resin composition includes silicon detected by a fluorescence X-ray at a rate of not less than 0.3 mass % and has a gel fraction of not less than 70%. A mass ratio of the silane compound to the free radical generating agent is not less than 35 and not more than 150.