PURPOSE: To provide a device to control the air-fuel ratio correctly by adjusting the fuel supply rapidly even with a gas engine of large fuel supply type.

CONSTITUTION: A fuel supply line 6 wherein the first supply pipe 6a provided with a mixing means 4 to mix the fuel and air and the second supply pipe 6b provided with a supply quantity adjusting means 9 to adjust the fuel supply to a combustion chamber 1 by the displacement of a control valve 23 are joined together on the upstream side of a throttle valve 5, is provided, and the drive of the control valve 23 is calculated according to the quantity of oxygen in the exhaust gas being discharged and at the same time, the fuel-air ratio is controlled by making a displacement of a control valve at a low speed after the control valve 23 is rapidly displaced. This constitution allows easy and exact fuel-air ratio to be controlled in a manner that mixing and fuel supply adjustment may be made without being affected by the intake pressure fluctuation downstream of the throttle valve. As the quantity of fuel supply can be adjusted by the displacement of the control valve, it is possible to realize an instantaneous control even with a large quantity of fuel supply, and the fuel supply can be controlled only by the quantity equivalent to over or short, leading to a compact supply control means.