Disclosed herein is a separator for an electrochemical element, comprising a fibrous structure, wherein the fibrous structure has a first fibrous layer part in which short fibers and/or pulp-like fibers are intertwined with each other, and a second fibrous layer part; some of the short fibers and/or the pulp-like fibers constituting the first fibrous layer part penetrates the second fibrous layer part; and a pore diameter distribution of the fibrous structure satisfies the following formula: 0 μm