PURPOSE: To supply mixture of stable air-fuel ratio to an engine by intermittent ly supplying air from an air supply source, mixing the supplied air and fuel in a mixing part, and connecting a surge tank accumulating air to a part be tween the air supply source and the mixing part.

CONSTITUTION: A surge tank 2 is interposed between a duty solenoid 1 as an air supply source and a branch pipe 5. Air is intermittently supplied from the duty solenoid 1, and the supplied air and fuel are mixed in each carburetor 3 of mixing part through the branch pipe 5. At this time, when negative pressure is generated in any cylinder 4, air in the surge tank 2 is supplied to the cylinder 4. As a result of this, a uniform quantity of air can be supplied to each pilot jet 32, and hence mixture of stable air-fuel ratio can be supplied to the engine.