Abstract: According to one embodiment, a separator is provided. The separator includes a composite membrane. The composite membrane includes a substrate layer, a first composite layer, and a second composite layer. The first composite layer is located on one surface of the substrate layer. The second composite layer is located on the other surface of the substrate layer. The composite membrane has a coefficient of air permeability of 1×10?14 m2 or less. The first composite layer has a first surface and a second surface. The first surface is in contact with the substrate layer. The second surface is located on an opposite side to the first surface. Denseness of a portion including the first surface is lower than denseness of a portion including the second surface in the first composite layer.