To provide a copper electrolyte for manufacturing a copper foil that satisfies characteristics required for an electrode for an on-board lithium ion battery, that is, has a smoothed surface, has a high tensile strength and an excellent elongation characteristic, is hardly annealed at an ordinary temperature, and causes small change in tensile strength with elapsed time.

The copper electrolyte contains the following (A) and (B) as additives, (A) is a compound obtained by the reaction of polyethyleneimine or a (poly) alkylene polyamine compound with an epoxy compound expressed by the following formula 1, (wherein A expresses a hydroxy group, an alkoxy group, a phenoxy group, a (meth)acryloxy group, -OCH₂CH=CH₂, or halogen), and (B) is an organic sulfur compound.