PURPOSE: To reconcile an improvement in drivability and an exhaust emission in the case where a lean combustion is carried out in time of light load but a theoretical air-fuel ratio combustion is performed in an area other than that, in an internal combustion engine equipped with an exhaust emission controlling three-way catalyst and an air-fuel ratio sensor in an exhaust passage.

CONSTITUTION: Gaseous fuel proportioned to an air quantity in a first passage 2 to be controlled by a throttle valve 4 operating according to the extent of accelerator opening is inducted into a venturi part 8 from a fuel passage 6. An air-fuel mixture in this first passage 2 is slightly richer than a theoretical air-fuel ratio. An air quantity in a second passage 3 is controlled by a throttle valve 5 interlocking with linkage including a link 21 in the throttle valve 4. At the time of light load, an integral mixing ratio is determined by a lever ratio between two levers 12 and 20 so as to become a lean air-fuel ratio. At the time of requiring the theoretical air-fuel ratio, a motor 17 is driven by a signal out of an air-fuel ratio sensor 26 and the throttle valve 5 is operated in the closing direction, thereby reducing the air quantity as specified.