A fuel additive contains ferrocene and/or ferrocene derivative(s), and lecithin. A fuel additive contains 80 to 99 mass % of ferrocene and/or ferrocene derivative(s), and 1 to 20 mass % of lecithin, and being in the solid state. A fuel additive contains 78 to 99 mass % of ferrocene and/or ferrocene derivative(s), 0.9 to 20 mass % of lecithin and 0.1 to 2 mass % of water, and being in the particle state. A fuel additive containing 2 to 5 mass % of ferrocene and/or ferrocene derivative(s), 5 to 50 mass % of lecithin and mineral oil, and being in the liquid state, wherein the ferrocene and/or ferrocene derivative(s), and the lecithin are dissolved in the mineral oil. The above-mentioned fuel additive is used by being added into a fuel so as to make the concentration of the ferrocene and/or ferrocene derivative(s) and the lecithin in ranges of 1 to 50 ppm, and 0.01 to 500 ppm, respectively.