To improve the stability of ignition and propagation ability of flame.

A method to suck air-fuel mixture in at least one combustion chamber 2 of a spark ignition internal combustion engine operated under a lean combustion condition has inflow of a flow of air through at least one intake manifold 1 with which a curved part situated upper stream from the combustion chamber is provided and inflow of a flow of gas fuel, having a velocity faster than that of a flow of air, in the combustion chamber 2 regarding a direction of a flow of air. Gas fuel points a flow of air, in which hydrocarbon is mixed, to the upper surface of the intake manifold 1 and is injected to the under surface side of the intake manifold 1 in a direction inclined based on the direction of the flow of air and to the vicinity of the combustion chamber 2 so that a liquid pressure part itself before inflow in the combustion chamber 2 is remarkably reduced.