A flexible high-frequency signal transmission line includes a dielectric body including laminated flexible dielectric layers. A signal line is provided in the dielectric body. A grounding conductor is arranged in the dielectric body to be opposed to the signal line via one of the dielectric layers. The grounding conductor is of a ladder structure including a plurality of openings and a plurality of bridges arranged alternately along the signal line. A characteristic impedance of the signal line changes between two adjacent ones of the plurality of bridges such that the characteristic impedance of the signal line rises from a minimum value to an intermediate value and to a maximum value and falls from the maximum value to the intermediate value and to the minimum value in this order.