In a transmission path transmitting high-frequency signals each signal contains a frequency component of over 8 GHz. The transmission path includes a nickel-phosphorus layer containing nickel and phosphorus, and a phosphorus concentration of the nickel-phosphorus layer is over 0 mass % and less than 8 mass %. Such a structure enables the transmission path to have little loss even when a signal at a frequency of over 8 GHz is transmitted.