An organic electroluminescence device comprising: a first electrode layer and a second electrode layer; a light-emitting layer containing an organic luminescent material and disposed between said first electrode layer and said second electrode layer; and a metal-based particle assembly layer under the first electrode layer, which first electrode layer is provided under the light-emitting layer, wherein the metal-based particle assembly layer is a layer consisting of a particle assembly including 30 or more metal-based particles separated from each other and disposed in two dimensions, said metal-based particles having an average particle diameter in a range of from 200 to 1600 nm, an average height, and an aspect ratio, as defined by a ratio of said average particle diameter to said average height, in a range of from 1 to 8, characterised in that: the average height is in a range of from 55 to 500 nm, an average distance between adjacent metal-based particles is in a range of from 1 to 150 nm, and the absorption plasmon peak at the highest wavelength of the absorption spectrum of visible light of the metal-based particle assembly layer shifts towards lower wavelengths, this wavelength shift being within the range 30-500 nm as compared with the absorption peak at the highest wavelength of the absorption spectrum of visible light of a reference metal-based particle assembly in which metal-based particles have a reference particle diameter equal to said average particle diameter, a reference height equal to said average height, a reference aspect ratio equal to said aspect ratio, made of a same reference material as the material of the metal-based particle assembly and wherein the particles are disposed such that each distance between adjacent metal-based particles of the reference metal-based particle assembly is in a range of from 1 to 2 μm. a first electrode layer and a second electrode layer; a light-emitting layer containing an organic luminescent material and disposed between said first electrode layer and said second electrode layer; and a metal-based particle assembly layer under the first electrode layer, which first electrode layer is provided under the light-emitting layer, wherein the metal-based particle assembly layer is a layer consisting of a particle assembly including 30 or more metal-based particles separated from each other and disposed in two dimensions, said metal-based particles having an average particle diameter in a range of from 200 to 1600 nm, an average height, and an aspect ratio, as defined by a ratio of said average particle diameter to said average height, in a range of from 1 to 8, characterised in that: the average height is in a range of from 55 to 500 nm, an average distance between adjacent metal-based particles is in a range of from 1 to 150 nm, and the absorption plasmon peak at the highest wavelength of the absorption spectrum of visible light of the metal-based particle assembly layer shifts towards lower wavelengths, this wavelength shift being within the range 30-500 nm as compared with the absorption peak at the highest wavelength of the absorption spectrum of visible light of a reference metal-based particle assembly in which metal-based particles have a reference particle diameter equal to said average particle diameter, a reference height equal to said average height, a reference aspect ratio equal to said aspect ratio, made of a same reference material as the material of the metal-based particle assembly and wherein the particles are disposed such that each distance between adjacent metal-based particles of the reference metal-based particle assembly is in a range of from 1 to 2 μm. the average height is in a range of from 55 to 500 nm, an average distance between adjacent metal-based particles is in a range of from 1 to 150 nm, and the absorption plasmon peak at the highest wavelength of the absorption spectrum of visible light of the metal-based particle assembly layer shifts towards lower wavelengths, this wavelength shift being within the range 30-500 nm as compared with the absorption peak at the highest wavelength of the absorption spectrum of visible light of a reference metal-based particle assembly in which metal-based particles have a reference particle diameter equal to said average particle diameter, a reference height equal to said average height, a reference aspect ratio equal to said aspect ratio, made of a same reference material as the material of the metal-based particle assembly and wherein the particles are disposed such that each distance between adjacent metal-based particles of the reference metal-based particle assembly is in a range of from 1 to 2 μm.