To enlarge buoyancy without increasing volume by coating the surface of a float with low surface energy material mainly composed of fluoroalkylsilane.

First, the surface of a float 7 is degreased with a solvent such as ethanol or the like and washed. Secondly, a coating liquid mainly composed of fluoroalkylsilane(FAS) is put in a dipping vessel to make the coating liquid adhere to the whole surface of the float 7. By using an air blow device or a centrifugal separator, coating liquid adhering to the surface of the float to excess is removed. Then, the float is put in a drying chamber for volatilizing a solvent in the adhering coating liquid to be dried. After that, the float 7 is put in an electric furnace and burnt below a heat resisting temperature. By the above steps, a fluoroalkylsilane coat film 14 is obtained on the surface of the float 7. Accordingly, a float of the same volume can be applied to an internal combustion engine having a larger displacement, and the accuracy of oil surface can be improved.