A separator comprising: a separator base material; and a particle-containing resin layer having a porous structure, wherein the particle-containing resin layer is provided on at least one main surface of the separator base material and contains particles and a resin, wherein a shape of the particles includes a plane, a plane rate of the particles is greater than 40% and less than 100%, measured by the method as set out in para. of the description, and a refractive index of the particles is equal to or greater than 1.3 and less than 2.4, wherein the particles are at least one kind of inorganic particles and organic particles, wherein the inorganic particles are at least one kind of particles selected from the group consisting of silicon oxide, zinc oxide, tin oxide, magnesium oxide, antimony oxide, aluminum oxide, magnesium sulfate, calcium sulfate, barium sulfate, strontium sulfate, magnesium carbonate, calcium carbonate, barium carbonate, lithium carbonate, magnesium hydroxide, aluminum hydroxide, zinc hydroxide, boehmite, white carbon, zirconium oxide hydrate, magnesium oxide hydrate, magnesium hydroxide octahy-drate, boron carbide, silicon nitride, boron nitride, aluminum nitride, titanium nitride, lithium fluoride, aluminum fluoride, calcium fluoride, barium fluoride, magnesium fluoride, tri-lithium phosphate, magnesium phosphate, magnesium hydrogen phosphate, ammonium polyphosphate, a silicate mineral, a carbonate mineral, and an oxide mineral, and the organic particles are at least one kind of particles selected from the group consisting of melamine, melamine cyanurate, melamine polyphosphate, crosslinked polymethyl methacrylate, polyethylene, polypropylene, polystyrene, polytetrafluoroethylene, polyvinylidene fluoride, polyamide, polyimide, a melamine resin, a phenol resin, and an epoxy resin. a separator base material; and a particle-containing resin layer having a porous structure, wherein the particle-containing resin layer is provided on at least one main surface of the separator base material and contains particles and a resin, wherein a shape of the particles includes a plane, a plane rate of the particles is greater than 40% and less than 100%, measured by the method as set out in para. of the description, and a refractive index of the particles is equal to or greater than 1.3 and less than 2.4, wherein the particles are at least one kind of inorganic particles and organic particles, wherein the inorganic particles are at least one kind of particles selected from the group consisting of silicon oxide, zinc oxide, tin oxide, magnesium oxide, antimony oxide, aluminum oxide, magnesium sulfate, calcium sulfate, barium sulfate, strontium sulfate, magnesium carbonate, calcium carbonate, barium carbonate, lithium carbonate, magnesium hydroxide, aluminum hydroxide, zinc hydroxide, boehmite, white carbon, zirconium oxide hydrate, magnesium oxide hydrate, magnesium hydroxide octahy-drate, boron carbide, silicon nitride, boron nitride, aluminum nitride, titanium nitride, lithium fluoride, aluminum fluoride, calcium fluoride, barium fluoride, magnesium fluoride, tri-lithium phosphate, magnesium phosphate, magnesium hydrogen phosphate, ammonium polyphosphate, a silicate mineral, a carbonate mineral, and an oxide mineral, and the organic particles are at least one kind of particles selected from the group consisting of melamine, melamine cyanurate, melamine polyphosphate, crosslinked polymethyl methacrylate, polyethylene, polypropylene, polystyrene, polytetrafluoroethylene, polyvinylidene fluoride, polyamide, polyimide, a melamine resin, a phenol resin, and an epoxy resin. <EOS>