PURPOSE: To prevent a flow rate adjusting needle valve from conflicting with an exhaust regulation even a user operates in what manner the flow rate adjusting needle valve for fuel, fed to a suction path from a fixed fuel chamber.

CONSTITUTION: A fuel passage is divided into a main fuel passage 6, having a variable orifice 8 for changing an effective area by a manual needle valve 9; and a sub-fuel-passage 11, having a fixed orifice 15. The variable orifice 8 is made to have a size capable of regulating a fuel flow rate for producing allowable high-density mixed gas by cooperating with the fixed orifice 15 at the time of full opening, and the fixed orifice 15 is made to have a size capable of giving a fuel flow rate for producing allowable lean mixed gas at the time of fully opening the variable orifice 8.