Abstract: An air-fuel ratio controlling system for an internal combustion engine including a control circuit for controlling, in accordance with a detected air-fuel ratio, the operation of a drive motor of a control valve mounted in an additional air supply passage. The control circuit includes a timing pulse generator for generating timing pulses synchronized with a running speed of the engine and thus the control valve is intermittently rotated by a predetermined amount in response to the timing pulses. By increasing the frequency of the timing pulses during the periods of rapid acceleration and deceleration, a response to the change in the engine speed is further improved.