A control point of a closed loop mixture control system for an internal combustion engine is varied by extending the duration of a signal representative of the deviation of air-fuel ratio from a preset value, or by alternately supplying an engine speed representative pulse sequence in opposite polarities to a first and a second output terminal. An integrator is connected to respond to the duration-extended deviation signal or connected to the first and second output terminals to effect integration of the alternatively supplied, opposite-polarity pulses to generate a stepwise voltage waveform. The integrator output is used as a feedback control signal which fluctuates about a level differing from the present value to shift the control point of the closed loop corresponding to said difference.