PURPOSE: To control sub-feedback, regardless of the gas richness or gas leanness of each bank, to suitably exclude NOx, etc., by applying single sub-feedback correcting quantity to the opening control for a bypass valve concerning the respective banks of a gas engine.

CONSTITUTION: In a V-shaped gas engine, respective main oxygen sensors 6 (L and R) are arranged respectively on the respective exhaust pipes 5 (L and R) of both right and left banks L and R. Also a sub-oxygen sensor 9 is arranged on the downstream side of a three-way catalyst 8, arranged on the downstream side in a joining place 7 of respective exhaust pipes 5. Moreover, the openings of respective bypass valves 4 (L and R) are determined by a control unit ECU based on respective outputs of all oxygen sensors 6 and 9, and air-fuel ratios of both the banks L and R are controlled independently, In this case, the respective sub-feedback correcting quantity of both the banks L and R are calculated and single sub-feedback correcting quantity is decided based on these calculated resultants, and then the decided resultant is applied to the air-fuel ratio control to both the banks L and R.