A high-frequency signal transmission line includes a dielectric element assembly including a plurality of dielectric layers laminated on each other, a linear signal line provided at the dielectric element assembly, and a first ground conductor provided on a first side in a direction of lamination relative to the signal line and including a plurality of openings arranged along the signal line. The dielectric layer positioned at an end of the first side in the direction of lamination includes an undulating portion provided on a first principal surface located on the first side in the direction of lamination, such that the undulating portion overlaps with the openings when viewed in a plan view in the direction of lamination.