To provide a small improving device for fuel oil capable of simply mounting without changing vehicle structure, mating members, piping or the like and useful for fuel economy and exhaust emission control.

A fuel inlet hole 713 existing on the top surface of a fuel filter 7, a fuel inlet hole 214 same as a fuel outlet bole 711, and a fuel outlet hole 231 are formed in the corresponding position on the top surface of a hollow box P, a screw part 221 same as a screw part 721 for fixing the fuel filter 7 to a mating member 8 is formed in the corresponding position, and a fixing part 131 to the fuel filter 7, a fuel inlet hole 123 and a fuel outlet hole 132 corresponding to the fuel inlet hole 713 and the fuel outlet hole 711 of the fuel filter 7 are formed on the bottom surface of the hollow box P, and a modifier 3 of fuel oil is filled in the hollow box P.