Provided is a separator including: a substrate; and a surface layer formed on at least one surface of the substrate, and having a higher porosity than that of the substrate. It is preferable that the surface layer includes: a first layer having convexities and concavities existing as cavities; and a second layer formed between the first layer and the separator, and the second layer has a higher porosity than that of the substrate, and the first layer has a higher porosity than that of the second layer. In this case, it is preferable that the porosity of the substrate is from 25% to 40%, the porosity of the first layer is from 60% to 90%, and the porosity of the second layer is from 40% to 65%.