The invention provides a surface-treated copper foil which can sufficiently ensure adhesive strength with a low-dielectric substrate used in forming a printed wiring board for high-frequency applications and can minimize transmission losses. There is provided a surface-treated copper foil for a low-dielectric substrate which is used in bonded relationship to a low-dielectric substrate, which is characterized in that a nodular-treated layer constituted by bump-like copper particles is formed on a surface of the copper foil and that ultrafine copper particles are caused to precipitate on the whole surface of the nodular-treated layer and adhere thereto and the roughness value Rz of the surface is 1.0 to 6.5 m. The surface color of the surface-treated copper foil has L* of not more than 50, a* of not more than 20 and b* of not more than 15. A passivated layer containing at least one kind selected from the group consisting of zinc and nickel is provided on surfaces of the ultrafine copper particles which are caused to precipitate on the whole of the surfaces of the bump-like copper particles of the nodular-treated layer and adhere thereto.