Provided are 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 of 200 to 1600 nm, an average height of 55 to 500 nm, and an aspect ratio, as defined by a ratio of the average particle diameter to the average height, of 1 to 8, the metal-based particle assembly having a scattering intensity ratio S1/S0 of not less than 0.15, S1 representing a scattering intensity at a maximum wavelength of a peak having a highest peak intensity in a scattering spectrum of light scattered at an angle of 30° from light incident on a surface of the metal-based particle assembly along a normal to the surface, S0 representing a scattering intensity of the peak for a reference system made of a barium sulfate layer, and an optical device including the assembly.