A main transmission line includes a substantially elongated dielectric body in which first and second substantially elongated signal conductors are disposed with a distance therebetween in a width direction. A first reference ground conductor and a first auxiliary ground conductor sandwich therebetween the first signal conductor in a thickness direction. A second reference ground conductor and a second auxiliary ground conductor sandwich therebetween the second signal conductor in the thickness direction. The second auxiliary ground conductor includes two substantially elongated conductors and a first bridge conductor, and the second auxiliary ground conductor includes two substantially elongated conductors and a second bridge conductor. The positions of the first and second bridge conductors in a lengthwise direction are different.