PURPOSE: To achieve complete combustion of lean mixture gas by arranging a supersonic atomizer for atomizing the fuel or combustion assisting agent then controlling on the basis of the detected level of combustion air.

CONSTITUTION: A supersonic atomizer 1 having a Piezo vibration chip 10 in the tank 11 is provided to lead in fuel from fuel tank 51 through fuel pump 50 thus to maintain the fuel level in said tank 11 at specific level through float valve mechanism. The Piezo vibration chip 10 is connected together with a diaphragm valve 30 to supersonic oscillator 14 through voltage regulator 35 for detecting the combustion air. Then the fuel mist produced through function of said Piezo vibration chip 10 is sucked through fuel supply tube 18 to a Venturi 20 by the negative pressure produced in the Venturi 20 where it is mixed with combustion air and fed to an internal-combustion engine. Under quick acceleration, fuel is incremented through power fuel feeding means 6.