Abstract: An air fuel ratio control system comprises a first valve provided in a fuel pipe through which a fuel is supplied to a burner, a second valve provided in an air pipe through which air is supplied to the burner, a varying circuit for varying responsive to a preset flow rate of fuel a valve opening degree of each of the first and second valves while maintaining a constant ratio between the valve opening degrees of the first and second valves, so that an air fuel ratio of a flow rate of fuel flowing through the first valve and a flow rate of air flowing through the second valve assumes a value greater than a theoretical air fuel ratio, a first sensor for detecting a flow rate of fuel flowing through the fuel pipe, a second sensor for detecting a flow rate of air flowing through the air pipe, a fuel bypass pipe provided in the fuel pipe for bypassing the first valve, a third valve provided in the fuel bypass pipe, and a control circuit supplied with output signals of the first and second sensors for controlling a