Unwanted radiation is reduced in a high-frequency signal transmission line that includes a ground conductor provided with an opening that overlaps a signal line. A dielectric element assembly has a relative dielectric constant ∈1 and has a first principal surface and a second principal surface. A signal line is provided in the dielectric element assembly. A ground conductor is provided in the dielectric element assembly and on the first principal surface side with respect to the signal line, faces the signal line, and is provided with an opening that overlaps the signal line. A high dielectric constant layer has a relative dielectric constant ∈2 higher than the relative dielectric constant ∈1 and is provided on the first principal surface so as to overlap the opening.