An exhaust system for an internal combustion engine comprising: a main exhaust passageway (2#1-2#4, 3a, 3b, 7, 8) including a plurality of upstream main exhaust passageways (2#1-2#4) in fluid communication with a respective one of a plurality of cylinders (#1-#4) of the internal combustion engine and a downstream main exhaust passageway, with the upstream main exhaust passageways (2#1-2#4) merging together to fluidly communicate with the downstream main exhaust passageway; first sensor means disposed in a bypass exhaust passageway (11#1-11#4, 14a, 14b, 16) of a bypass catalytic converter to sense air-fuel ratio of an exhaust gas flowing into the bypass catalytic converter; and second sensor means disposed in the bypass exhaust passageway (11#1-11#4, 14a, 14b, 16) downstream of the bypass catalytic converter to sense air-fuel ratio of the exhaust gas flowing out of the bypass catalytic converter, characterised by the bypass exhaust passageway (11#1-11#4, 14a, 14b, 16) including a plurality of upstream bypass exhaust passageways (11#1-11#4) in fluid communication with the upstream main exhaust passageways (2#1-2#4) so as to bypass portions thereof between upstream junctions (12#1-12#4) at which the upstream bypass exhaust passageways (11 #1-11 #4) branch out of respective ones of the upstream main exhaust passageways (2#1-2#4) and a downstream junction at which a downstream bypass exhaust passageway merges into the downstream main exhaust passageway, the upstream main exhaust passageways (2#1-2#4) converging downstream of the upstream junction (12#1-12#4); the bypass catalytic converter provided in the bypass exhaust passageway (11#1-11#4, 14a, 14b, 16); valve means provided in the portions of the upstream main exhaust passageway (2#1-2#4) and configured to open or close the portions of the upstream main exhaust passageway (2#1-2#4). a main exhaust passageway (2#1-2#4, 3a, 3b, 7, 8) including a plurality of upstream main exhaust passageways (2#1-2#4) in fluid communication with a respective one of a plurality of cylinders (#1-#4) of the internal combustion engine and a downstream main exhaust passageway, with the upstream main exhaust passageways (2#1-2#4) merging together to fluidly communicate with the downstream main exhaust passageway; first sensor means disposed in a bypass exhaust passageway (11#1-11#4, 14a, 14b, 16) of a bypass catalytic converter to sense air-fuel ratio of an exhaust gas flowing into the bypass catalytic converter; and second sensor means disposed in the bypass exhaust passageway (11#1-11#4, 14a, 14b, 16) downstream of the bypass catalytic converter to sense air-fuel ratio of the exhaust gas flowing out of the bypass catalytic converter, characterised by the bypass exhaust passageway (11#1-11#4, 14a, 14b, 16) including a plurality of upstream bypass exhaust passageways (11#1-11#4) in fluid communication with the upstream main exhaust passageways (2#1-2#4) so as to bypass portions thereof between upstream junctions (12#1-12#4) at which the upstream bypass exhaust passageways (11 #1-11 #4) branch out of respective ones of the upstream main exhaust passageways (2#1-2#4) and a downstream junction at which a downstream bypass exhaust passageway merges into the downstream main exhaust passageway, the upstream main exhaust passageways (2#1-2#4) converging downstream of the upstream junction (12#1-12#4); the bypass catalytic converter provided in the bypass exhaust passageway (11#1-11#4, 14a, 14b, 16); valve means provided in the portions of the upstream main exhaust passageway (2#1-2#4) and configured to open or close the portions of the upstream main exhaust passageway (2#1-2#4).