To provide a method for producing an emulsifier-free fuel oil stabilized for a long period, capable of simply producing the fuel oil at a low cost with a small load on the environment, by mixing an oil and water without using an emulsifier.

This method for producing the emulsifier-free fuel oil comprises continuously feeding a mixed gas of ozone reactive with an organic substance and solubilized in water and air, so as to produce the emulsifier-free fuel oil which is formed by mixing the oil and the water and bonding them to each other more strongly when compared with an emulsified oil of the water and the oil given by utilizing the emulsifier.