PURPOSE: To provide a method of controlling the air fuel ratio of an internal combustion engine, wherein when it is in the predetermined second running state, an average value of correction for air fuel ratio is calculated and memoriezed, and said average value is used as the original value for correction value for air fuel ratio when control for the closed loop is resumed, so that an optimum value is attained within a short period.

CONSTITUTION: A microcomputer (not shown) is incorporated in a control circuit 28 to ascertain that the engine is in the predetermined second running state and to calculate the average value Fi(A/F) of correction factor for air fuel ratio when it is in the second running state. When the engine has not been in the first running state, it is fixed to a position equal to the average value Fi(A/F), and the control of open loop is carried out. When the control of closed loop is resumed, the original value of the correction air fuel ratio f(A/F) is controlled to be equal to this average value Fi(A/F), the average value Fi(A/F) being renewed only when it has become in the second running state. Thus, the optimum value can be attained within a short period, and the running characteristics and exhaust gas purifying characteristics can be improved.