In a coaxial cable comprising a cylindrical conductor tube structure, a cylindrical core conductor arranged within the conductor tube structure, and a dielectric member provide between the conductor tube structure and the core conductor, the conductor tube structure is constituted by a cylindrical outer conductor and a cylindrical inner conductor provided within the outer conductor in coaxial and radially spaced relationship with the outer conductor to be capacitively coupled with the outer conductor. The inner conductor has a first axial end portion protruded axially outwardly from one end of the outer conductor and a second axial end portion received in the outer conductor retracted axially inwardly from the other end of the outer conductor. The outer and inner conductors of the conductor tube structure are thermally insulated from each other thereby reducing heat transmission between the outer and inner conductors without lessening conductivity for high frequency signals.