A high-frequency signal transmission line includes a dielectric body, a signal conductor, and a ground conductor. The dielectric body extends along a high-frequency signal transmission direction. The signal conductor is in the dielectric body and extends along the high-frequency signal transmission direction. The ground conductor is in the dielectric body and is electromagnetically coupled to the signal conductor. The dielectric body includes, along the high-frequency signal transmission direction, a plurality of straight portions and a curved portion connecting the plurality of straight portions. In the curved portion, the signal conductor is located at a position on an inner side of a curve relative to a center position in a width direction of the dielectric body.