A separator for an electric double-layer capacitor is provided having a double-layer structure made of a fibrous layer A obtained by papermaking using a Fourdrinier net or a tanmo net, and a fibrous layer B obtained by papermaking using a cylinder net. The fibrous layer A is a layer that is refined until a CSF value decreases once to 0 ml (lower limit), and further refined until it turns to rise to 10 to 600 ml, the fibrous layer B is a layer that is refined until a CSF value of 700 to 0 ml, and the fibrous layer A and the fibrous layer B contain 70% by mass or more refinable, regenerated cellulose fibers. The density of the entire double-layer structure is 0.25 to 0.65 g/cm3, and the thickness of the same is 10 to 150 μm. This allows provision of a separator for an electric double-layer capacitor having excellent tensile strength and tear strength, provision of a degree of mechanical strength such that the separator does not break in a manufacturing process of a wound or a stacked type electric double-layer capacitor, and improvement of productivity without adversely affecting internal resistance and leakage current properties of the capacitor as an energy storage device.