The present invention provides an organic/inorganic composite containing a rare earth metal or/and Period IV transition metal in which the aforementioned rare earth metal or/and Period IV transition metal is doped at a high concentration, and control of quenching and optical transparency are assured thereby; and an optical amplifier, a light control optical element, and luminescent device utilizing the same. The organic/inorganic composite containing a rare earth metal or/and Period IV transition metal is one in which at least one species of rare earth metal or/and Period IV transition metal is dispersed in an organic polymer, with the aforementioned composite containing an optically transparent organic polymer and an inorganic dispersion phase comprising: (1) a rare earth metal and (2) another element coordinated thereto via an oxygen atom(s). The inorganic dispersion phase in which another metal coordinates to rare earth metal or/and Period IV transition metal via an oxygen atom(s) preferably has an average particle size ranging from 0.1 to 1000 nm. Preferably, the ratio of rare earth metal or/and Period IV transition metal when mathematically converted to solid content is 90 mass % or less of the total mass of the organic polymer and the rare earth metal dispersion phase.