A fuel additive to gasoline and diesel fuels which when added in small quantities relative to total volume of fuel treated and burned as fuel in an internal combustion engine, enhances fuel burning to concurrently increase mileage and reduce emissions. A sound duct is integrally formed with the inner frame, and a wiring board is contained in the casing and arranged In the vicinity of a bottom wall portion of the casing. The electrical control device is energized by a switch.