A three-dimensional structure of two carbon nanotubes (CN) joined by a connecting region containing a pentagon and heptagon pair is given by the use of a projection method. The connecting joint is uniquely determined for the given two chiral vectors of CN by a vector which defines a three-dimensional dihedral angle. The tunneling conductance is calculated for a metal-metal CN junction and a metal-semiconducting CN junction. The calculated results clearly show that these junctions work as the smallest semiconductor devices.