The oxygen sensors detect the level of oxygen concentration in the exhaust gases. A total of four front and rear oxygen sensors are installed in front and back of the dual manifold catalytic converter (MCC), respectively. The Engine-A/T-ECU uses the output signal of the front oxygen sensor for each MCC to effect closed-loop control, and uses the output signal of the rear oxygen sensor in order to make corrections to the output signal of the front oxygen sensor. By resolving the output signal deviations associated with the deterioration of the front oxygen sensor, this system is able to effect a level of exhaust gas control that is higher than in the past.