A transmission line device includes first and second transmission lines. The first transmission line includes a first electrode pad that is electrically connected to a first signal conductor pattern, and a second electrode pad and a third electrode pad that are portions of a first ground conductor pattern. The second transmission line includes a fourth electrode pad that is electrically connected to a second signal conductor pattern, and a fifth electrode pad and a sixth electrode pad that are portions of a second ground conductor pattern. The first electrode pad is between the second electrode pad and the third electrode pad, and the fourth electrode pad is between the fifth electrode pad and the sixth electrode pad. The second electrode pad and the third electrode pad are larger than the first electrode pad, and the fifth electrode pad and the sixth electrode pad are larger than the fourth electrode pad.