A fuel supply system for use in an internal combustion engine equipped with a three way catalytic converter which reduces the amount of NO.sub.x and oxidizes CO and HC at the same time, for controlling air fuel ratio by using an electronic circuit. According to this fuel supply system, when the internal combustion engine is in the low load running mode, the air fuel ratio of the mixture charge to be supplied to the engine is increased. In addition, when cooling water in the internal combustion engine remains above a given temperature and the load of the engine is over a given level, then the mixture charge is maintained at an increased air fuel ratio (a lean mixture charge) rather than at the stoichiometric air fuel ratio. In addition, there is provided means for slowing down a transient phase from the stoichiometric air fuel ratio to the increased air fuel ratio or vice versa.