In an in-line four-cylinder internal combustion engine, exhaust ports (2b, 2c) of cylinders #2 and #3 merge together inside a cylinder head and become open as one flat collective exhaust port (2bc). An exhaust manifold (5) includes separate individual exhaust pipes (6, 7) for cylinders #1 and #4 and a collective exhaust pipe (8) for cylinders #2 and #4. Tip ends of these three exhaust pipes are connected to a catalytic converter (11). An equivalent diameter of the collective exhaust port (2bc) is larger than equivalent diameters of the exhaust ports (2a, 2d) before merging. A short diameter of the collective exhaust port (2bc) is smaller than or equal to the equivalent diameters of the exhaust ports (2b, 2c).