A differential transmission path composed of a pair of transmission lines is formed on an upper surface of a base insulating layer. A ground conductor layer is formed on a lower surface of the base insulating layer. The ground conductor layer is opposite to the differential transmission path with the base insulating layer sandwiched therebetween. A spacing between the transmission lines at a part of the differential transmission path is smaller than a spacing between the transmission lines at another part of the differential transmission path. A thickness of a part of the ground conductor layer overlapping the part of the differential transmission path is smaller than the thickness of another part of the ground conductor layer overlapping the another part of the differential transmission path.