An FPC board includes a base insulating layer. A plurality of wiring traces are formed on the base insulating layer. The adjacent wiring traces are arranged at a distance d from each other, and each wiring trace has a predetermined width and a thickness t1. Each transmission line pair is constituted by the two adjacent wiring traces of the plurality of wring traces. A ratio of the thickness t1 of the wiring trace to the distance d between the adjacent wiring traces is set to 0.8 or more. A cover insulating layer may be formed on the base insulating layer to cover the wiring traces. A metal layer having a predetermined thickness may be provided on a back surface of the base insulating layer. Furthermore, a differential impedance of each transmission line pair may be set to 100 Ω.