To maintain the theoretical air/fuel ratio of a mixed fuel, which comprises a hydrocarbon and at least one selected from alcohols and ethers, always at a constant value in the manufacture of the mixed fuel.

In the manufacture of the mixed fuel comprising the hydrocarbon and at least one additive fuel selected from alcohols and ethers, a paraffinic hydrocarbon is used as the hydrocarbon, and a relationship between the amount of the additive fuel and the theoretical air/fuel ratio of the mixed fuel is determined by using the paraffinic hydrocarbon as a parameter. From this relationship, the amount of the additive fuel and the theoretical air/fuel ratio of the mixed fuel, at a point in which the theoretical air/fuel ratios intersect, are determined. Thus, a mixed fuel having a fixed theoretical air/fuel ratio is manufactured.