To provide a new type of additive for producing oil/water mixed fuel that can completely homogenize water into oil, differing from the conventional emulsion fuel, and causes no phase separation even when the mixed fuel is allowed to stand for several years.

Oleic acid is used as a main component for the additive for oil/water mixed fuel and further polyoxyethylene sorbitan mono-oleate and triethanolamine are also added thereto.