This surface-treated copper foil is characterized in that the amount of adhesion of Si on the copper foil surface is from 3.1 to 300 μg/dm2, and the amount of adhesion of N on the copper foil surface is from 2.5 to 690 μg/dm2. The objective of the present invention is to obtain a copper foil having improved peel strength in providing a copper foil for a flexible printed substrate (FPC), in which a copper foil is laminated to a liquid crystal polymer (LCP) suitable for high-frequency applications.