In a differential transmission line, a substrate has first and second surfaces parallel to each other, and a first grounding conductor is formed on the second surface of the substrate. A dielectric layer is formed on the first grounding conductor, and a second grounding conductor is formed on the dielectric layer. First and the second signal conductors are formed to be parallel to each other on the first surface of the substrate. The first signal conductor and the first and second grounding conductors constitute a first transmission line, and the second signal conductor and the first and second grounding conductors constitute a second transmission line. A slot is formed in the first grounding conductor to three-dimensionally intersect with the first and second signal conductors and to be orthogonal to a longitudinal direction thereof, and a connecting conductor is formed for connecting the first grounding conductor with the second grounding conductor.