Abstract: [Object] To provide an additive that can achieve uniform dispersion of oil and water to improve combustion efficiency and also can stabilize the uniform dispersion of oil and water for a long period of time, an additive-modified fuel oil and a water-mixed fuel obtained by addition of the additive, and methods of producing them. [Solution] A method of producing an additive that is used for generating a water-mixed fuel or a water-unmixed modified fuel oil by being added to a fuel oil includes the steps of: dissolving a relatively low molecular weight alcohol in a fatty acid or a relatively high molecular weight alcohol to generate an intermediate product 1; adding a surfactant to the intermediate product 1 to generate an intermediate product 2; adding only an amine compound or both an amine compound and ethylene glycol to the intermediate product 2 to generate an intermediate product 3; and adding aqueous ammonia or gaseous ammonia to the intermediate product 3 to generate a desired additive.