An object of the present invention is to provide a copper foil excellent in softening resistance performance which reduces decrease in tensile strength after heat treatment at about 350° C. to 400° C. In order to achieve the object, a surface-treated copper foil provided with a rust-proofing treatment layer on both surfaces of a copper foil in which a rust-proofing treatment layer is constituted by zinc alloy, and the either rust-proofing treatment layer is a zinc alloy layer having zinc amount of 20 mg/m2 to 1,000 mg/m2; and the copper foil contains one or two or more of small amount elements selected from carbon, sulfur, chlorine and nitrogen, and a sum amount thereof is 100 ppm or more is adopted.