Provided are a method and a device for the stable production of fuel with which fuel efficiency can be improved and the generation of harmful substances can be easily inhibited. The method and device for producing fuel improve the level of satisfaction of the user and contribute to preventing environmental damage. By means of the method for producing fuel, a natural botanical enzyme containing at least lipase is mixed with water and the resulting aqueous enzyme is mixed and reacted with a petroleum-based hydrocarbon oil to produce a fuel oil. The natural botanical enzyme further comprises cellulase. Methanol is further mixed with the aqueous enzyme.
