A high-frequency signal transmission line includes a dielectric body including a plurality of dielectric sheets. A signal line is provided in the dielectric body. A connector is mounted on a first main surface of the dielectric body and electrically connected to the signal line. A ground conductor is provided on a second main surface side of the dielectric body, compared with the signal line, and faces the signal line across the dielectric sheet. In the ground conductor, conductor-missing portions are provided in which no conductors are provided in at least portions of regions overlapping with the signal line in planar in connection portions. Adjustment conductors are provided in the second main surface of the dielectric body, and overlap with at least portions of the conductor-missing portions in the planar view.