One of the objects of the present invention is to provide a separator for an electrochemical device, capable of suppressing an increase in a resistance value of a storage element.In accordance with one aspect of the present invention, a separator 16c for an electrochemical device is formed such that plural high porosity portions 16c1 from an upper surface to a lower surface in a thickness direction thereof and plural low porosity portions 16c2 from the upper surface to the lower surface in the thickness direction thereof are arranged in a region which is interposed between a positive electrode 16a and a negative electrode 16b.