In a transmission line, a signal conductor layer extends in a front-back direction orthogonal to an up-down direction. A ground conductor layer is above the signal conductor layer. When viewed in a first orthogonal direction, first hollow portions are arranged in the front-back direction in a first direction of the signal conductor layer, and second hollow portions are arranged in the front-back direction in a second direction of the signal conductor layer. Each of regions between adjacent first hollow portions in the front-back direction is a first region. Each of regions between adjacent second hollow portions in the front-back direction is a second region. Each of the first hollow portions overlaps with a corresponding one of the second regions when viewed in a second orthogonal direction. Each of the second hollow portions overlaps with a corresponding one of the first regions when viewed in the second orthogonal direction.