PURPOSE: To positively and easily reduce vibration of air-fuel ratios by equalizing the feedback periods of air-fuel ratio control in two systems, and shifting the phases in feedback in the two systems 180 degrees one relative to the other.

CONSTITUTION: Intake passages 1-1, 1-2, engines E-1, E-2, and an exhaust passage 10 are provided respectively in two systems. First oxygen sensors 12-1, 12-2 are arranged on the upstream side of a three way catalytic converter 11, and a second oxygen sensor 13 is arranged in the downstream of the three way catalytic converter 11. Further, an air-fuel ratio control device 15 is provided to respond to outputs of the first and second oxygen sensors 12-1, 12-2, 13, and to control the air-fuel ratio of the respective engines independently. Furthermore, the feedback periods of air-fuel ratio control in the engines E-1, E-2 in the two systems are equalized so as to shift the phases in feedback in the two system about 180 degrees one relative to the other. Thus, vibration of air-fuel ratios in the respective systems are negated mutually, and thus the vibration of air-fuel ratios is extremely reduced.