A resin composition is made of a mixture including a crosslinked polyethylene and a non-crosslinked polyethylene. The mixture has a density of 0.960 g/cm3 or more and a melt fracture tension within a range from 20 to 100 mN. A high-frequency co-axial cable includes sequentially: an internal conductor; an internal solid layer; a foamed insulation layer; an external solid layer; an external conductor; and an outer coat. The foamed insulation layer of includes a foam body of the above resin composition.