There is provided a metal-based particle assembly comprising 30 or more metal-based particles separated from each other and disposed in two dimensions, the metal-based particles having an average particle diameter in a range of from 200 to 1600 nm, an average height in a range of from 55 to 500 nm, and an aspect ratio, as defined by a ratio of the average particle diameter to the average height, in a range of from 1 to 8, wherein the metal-based particle assembly has in an absorption spectrum for a visible light region a maximum wavelength of a peak at a longest side in wavelength, and an absorbance at the maximum wavelength is higher as compared with that of a reference metal-based particle assembly, on the premise that the numbers of the metal-based particles are the same. The metal-based particle assembly of the present invention presents significantly intense plasmon resonance.