Abstract: An automotive engine has a canister for purging fuel vapor to an intake passage of the engine through a purge valve. An O.sub.2 -sensor is provided for producing an output voltage relative to oxygen concentration of exhaust gases of the engine. A feedback control system responds to an output signal of the O.sub.2 -sensor for controlling the air-fuel ratio of mixture supplied to the engine with a correcting coefficient. Stopping of the purging of the fuel vapor at idling of the engine is detected and a purge cut-off signal is produced. In response to the purge cutt-off signal, the coefficient is increased to 1.0, so that the air-fuel ratio is reduced to enrich the mixture, thereby preventing the mixture from becoming lean at the stopping of the purging.