The purpose of the present invention is to provide a surface-treated copper foil provided with a roughened layer, said surface-treated copper foil exerting no skin effect of the roughened layer in high frequency signal transmission and thus enabling the formation of a circuit capable of achieving a designed signal transmission speed. To solve this problem, provided is a surface-treated copper foil for forming a high frequency signal transmission circuit, said surface-treated copper foil being provided with a roughened layer on the surface of a copper foil, characterized in that the roughened layer has needle- or plate-like minute projections and recesses formed of a copper complex compound containing copper oxide and cuprous oxide, and, when a cross section of the copper foil is observed, the average crystal size is 2.5 μm or greater.
