An internal combustion engine having an air-fuel mixture supply controller is provided with an air-to-fuel ratio detector for detecting the oxygen content in the exhaust gas of the engine and a feedback controller for correcting mixture supply operation of the mixture supply controller in response to the oxygen content. A halt circuit and a hold circuit, being responsive to first and second operating conditions of the engine respectively, are connected to the feedback controller to halt and hold the feedback control, thus causing the mixture supply controller to supply the richer or leaner mixture as desired.