A high-frequency transmission line includes a multi-layer substrate, a plurality of line portions, a plurality of signal vias, a ground plane, and a plurality of metal portions. When a high-frequency signal that is transmitted through each of the plurality of line portions is transmitted through the transmission area corresponding to each of the plurality of line portions, the plurality of metal portions confine the high-frequency signal in the transmission area. The plurality of metal portions pass through at least a single dielectric layer, are electrically continuous with the ground plane. The plurality of metal portions are formed along the transmission direction and are arranged in an alternating manner with each of the plurality of signal vias so as to sandwich each of the plurality of signal vias in a direction differing from the transmission direction.