To provide a gas oil additive which can effectively reduce nitrogen oxides to be generated upon combustion of gas oil.

Ultrafine carbon black particles 1 having a particle size of 0.05-0.15 μm are added to gas oil such that the amount of the ultrafine carbon black particles 1 come to 0.001-0.5 wt.% based on the gas oil. The addition of the ultrafine carbon black particles 1 inhibits the bonding of nitrogen to oxygen and, at the same time, causes the deoxygenation of nitrogen oxides.