To provide an exhaust system of a motorcycle for securing layout arrangement suitable for a service condition of an oxygen sensor without impairing an appearance.

This exhaust system includes an engine 7 mounted between a pair of right-left main frames 3 and 3, an exhaust collecting pipe 26 for connecting exhaust pipes 25, 25, etc. extended backward to a vehicle body lower part from the engine 7 and an exhaust chamber 28 and biasedly arranged to one side in the vehicle width direction more than the vehicle body center line Q in the vehicle body longitudinal direction, and an oil pan 30 separately arranged in front of a vehicle body more than the exhaust chamber 28 and storing engine oil. The oil pan 30 is formed with an oil storage part 30a swollen downward to the vehicle body so as to sandwich the vehicle body center line Q between the exhaust collecting pipe 26 and itself and separately opposed in the vehicle body longitudinal direction to a part of the exhaust chamber 28, and the oxygen sensor 37 is arranged between the oil storage part 30a and a part of the exhaust chamber 28.

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