A separator comprising a composite membrane that includes: a substrate layer of a nonwoven fabric or a self-supporting porous membrane; a first composite layer located on one surface (S1) of the substrate layer ; and a second composite layer located on the other surface (S2) of the substrate layer , wherein the composite membrane has a coefficient of air permeability of 1 × 10 -14 m 2 or less, the coefficient of air permeability being determined according to the method indicated in the description, the first composite layer and the second composite layer each includes inorganic solid particles and a polymeric material , the polymeric material has a portion formed of a monomer unit containing a hydrocarbon having a functional group containing at least one element selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F), and a proportion occupied by the portion formed of a monomer unit is 70 mol% or more, the first composite layer has a first surface (FS C1 ) in contact with the substrate layer and a second surface (SS C1 ) located on an opposite side to the first surface (FS C1 ), and wherein a ratio SD C1 /FD C1 of a second density SD C1 to a first density FD C1 is 1.03 or more in the first composite layer , the first density FD C1 of the first composite layer is a proportion occupied by a portion other than holes in a region from the first surface (FS C1 ) to a surface (M1) located at a depth of 0.2 T C1 with respect to a thickness T C1 of the first composite layer and the second density SD C1 of the first composite layer is a proportion occupied by a portion other than holes in a region from the second surface (SS C1 ) to a surface (M2) located at a depth of 0.2 T C1 with respect to the thickness T C1 of the first composite layer , the first density FD C1 of the first composite layer and the second density SD C1 of the first composite layer being determined according to the method indicated in the description,. a substrate layer of a nonwoven fabric or a self-supporting porous membrane; a first composite layer located on one surface (S1) of the substrate layer ; and a second composite layer located on the other surface (S2) of the substrate layer , wherein the composite membrane has a coefficient of air permeability of 1 × 10 -14 m 2 or less, the coefficient of air permeability being determined according to the method indicated in the description, the first composite layer and the second composite layer each includes inorganic solid particles and a polymeric material , the polymeric material has a portion formed of a monomer unit containing a hydrocarbon having a functional group containing at least one element selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F), and a proportion occupied by the portion formed of a monomer unit is 70 mol% or more, the first composite layer has a first surface (FS C1 ) in contact with the substrate layer and a second surface (SS C1 ) located on an opposite side to the first surface (FS C1 ), and wherein a ratio SD C1 /FD C1 of a second density SD C1 to a first density FD C1 is 1.03 or more in the first composite layer , the first density FD C1 of the first composite layer is a proportion occupied by a portion other than holes in a region from the first surface (FS C1 ) to a surface (M1) located at a depth of 0.2 T C1 with respect to a thickness T C1 of the first composite layer and the second density SD C1 of the first composite layer is a proportion occupied by a portion other than holes in a region from the second surface (SS C1 ) to a surface (M2) located at a depth of 0.2 T C1 with respect to the thickness T C1 of the first composite layer , the first density FD C1 of the first composite layer and the second density SD C1 of the first composite layer being determined according to the method indicated in the description,. <EOS>