An in-line four-cylinder internal combustion engine wherein exhaust ports (8, 9) of a pair of cylinders positioned at the center are connected to a single merged exhaust port (11) and this merged exhaust port (11) is connected through a first exhaust pipe (19) to a catalytic converter (18). On the other hand, exhaust ports (7, 10) of pairs of cylinders positioned at the two ends are connected through a second exhaust pipe (21) to a catalytic converter (18). A length of the first exhaust pipe (19) from the merged exhaust port (11) to the catalytic converter (18) is made shorter than the lengthof the second exhaust pipe (21) from the exhaust ports (7, 10) to the catalytic converter (18).