A dielectric element assembly includes a plurality of stacked dielectric sheets. A signal line is provided in or on the dielectric element assembly. A ground conductor is provided in or on the dielectric element assembly on the negative direction side of a z-axis direction relative to the signal line and is arranged so as to oppose the signal line via the dielectric sheets. The ground conductor includes a main body portion and protruding portions. The main body portion extends along the signal line on one side of a direction perpendicular or substantially perpendicular to the signal line relative to the signal line when viewed in plan from the z-axis direction. The protruding portions protrude from the main body portion toward the signal line and overlap the signal line when viewed in plan from the z-axis direction.