To provide a fuel additive for preventing a slagging trouble caused by the ash content in various fuels in the combustion of especially a fuel having a high ash content represented by coal, oil coke, etc., and provide a combustion method of the fuel using the fuel additive.

The fuel additive is composed of a composition produced from ultrafine silica sol particles having a particle diameter of 3-200 nm and ultrafine Al(OH)3 particles and/or ultrafine Zr compound particles surface-modified with a surfactant and having a particle diameter of 2-100 nm by dispersing the Al(OH)3 particles and the Zr compound particles in water. The fuel additive is added to a liquid fuel oil or a solid fuel and the fuel is burned.