Abstract: An exhaust gas passageway of an internal combustion engine is basically provided with a main exhaust passageway, a bypass exhaust passageway, a bypass catalytic converter provided in the bypass exhaust passageway, a bypass control valve, an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor. The bypass exhaust passageway bypasses a portion of the main exhaust passageway between an upstream junction and a downstream junction. The bypass control valve opens or closes a portion of the main exhaust passageway. The upstream air-fuel ratio sensor is disposed upstream of the bypass catalytic converter to sense air-fuel ratio of an exhaust gas flowing into the bypass catalytic converter. The downstream air-fuel ratio sensor is disposed downstream of the bypass catalytic converter to sense air-fuel ratio of the exhaust gas flowing out of the bypass catalytic converter.