To provide an electrolytic copper foil whose tensile strength at normal temperature is 650 MPa or higher, tensile strength after heat treatment of 300°C×1 hour is 450 MPa or the higher, and electric conductivity is 80% or higher, which are achieved by fetching W into the copper foil as a copper alloy, and to provide the copper foil which holds adhesion of the collector and the collector (copper foil) with large expansion and construction by a polyimide binder so as not to cause breaking of the collector (copper foil).

The copper alloy foil contains 0.008-0.020 wt.% of tungsten (W), and has the tensile strength at normal temperature of 650 MPa or higher, the tensile strength after the heat treatment of 300°C×1 hour of 450 MPa or higher, and the electrical conductivity of 80% or higher. The copper alloy foil has an elongation at normal temperature of 2.5% or higher and an elongation after 300°C×1 hour of 3.5% or higher.