A method of and a device for regulating air-fuel ratio of an internal combustion engine includes an oxygen or lambda probe and a signal processing unit connected to the lambda probe. The change-overs of the lambda signal at the output of the lambda probe are compared with a succession of adjustable first time intervals t.sub.1 and, when no change-over is detected during the first time interval, the output signal from the signal processing unit which controls the air-fuel ratio is abruptly changed in a jump. To avoid overshoot of the output control signal, there is provided a resetting unit by means of which the jump of the control signal is reversed when the change-over of the lambda signal occurs in an additional monitoring time interval t.sub.2 following the first monitoring time interval t.sub.1.