INTERNAL COMBUSTION ENGINE EXHAUST PIPE

PROBLEM TO BE SOLVED: To realize an internal combustion engine exhaust pipe capable of solving a problem that water is accumulated in a lower portion of a junction part between the exhaust pipe and a cylinder head with a simple configuration.SOLUTION: In an internal combustion engine exhaust pipe 41 generally horizontal in a coupled portion with a cylinder head 1b and including a catalyst 42 provided halfway and a front Osensor provided upstream of this catalyst 42 and serving as an air-fuel ratio sensor, at least a region provided with a flange 410 and an inner bottom surface 41x of a horizontal portion 411 that are upstream end portions are formed into a valley shape inclined downward toward a valley bottom 41y set at a predetermined width-direction position, and the valley bottom 41y is inclined downward toward a downstream side.