PURPOSE: To make signal transmission speed higher than usual by arranging conductors on both sides of the hollow portion of a tubular insulated layer so as to make the equivalent dielectric constant between the conductors as low as nearly equal to a gas dielectric constant.

CONSTITUTION: Conductors 13, 14 of a cable W10 are arranged so as to touch the inside circumference of an insulating layer 11 formed into a tubular shape and to be along a hollow portion 12. The insulating layer 11 is formed, for instance, of PE, PP or the like and is foamed as much as it can hold strength against external force so as to reduce a dielectric constant. Then the equivalent dielectric constant between the conductors 13, 14 becomes as low as nearly equal to a gas dielectric constant, and the dielectric constant of the insulating layer itself scarcely influences the dielectric constant between the conductors. The sheath 17 is formed into a square shape in its cross section of a thermoplastic resin such as PVC, and their surfaces are connected together by fusion so as to be integrated when it is necessary. Thereby a signal transmission speed can be increased and the strength of the insulating layer can be secured.