An exhaust system, comprising: a first exhaust pipe connected to an engine; an enlarged portion connected to the first exhaust pipe; a housing portion connected to the enlarged portion; a catalyst arranged inside of the housing portion; a second exhaust pipe connected to the housing portion; and a muffler connected to the second exhaust pipe; the first exhaust pipe including a curved portion, and a straight portion extending linearly from the curved portion, the enlarged portion including an upstream end connected to the straight portion, and a downstream end connected to the housing portion, the enlarged portion forming a flow path that enlarges from the upstream end of the enlarged portion to the downstream end of the enlarged portion, and the straight portion being inserted from outside of the enlarged portion into inside of the enlarged portion through the upstream end of the enlarged portion, and extending toward the catalyst; whereby the curved portion has a central axis passing through the center of a flow path in the curved portion, the curved portion includes an inner curved portion positioned more inward than the central axis of the curved portion, having a U-shaped cross sectional face, and extending along the central axis of the curved portion, and an outer curved portion positioned more outward than the central axis of the curved portion, having a U-shaped cross sectional face, and extending along the central axis of the curved portion, characterised in that the straight portion includes an inner straight portion continuing to the inner curved portion, and an outer straight portion continuing to the outer curved portion, and a length of the outer straight portion is larger than a length (Gc) of the inner straight portion. a first exhaust pipe connected to an engine; an enlarged portion connected to the first exhaust pipe; a housing portion connected to the enlarged portion; a catalyst arranged inside of the housing portion; a second exhaust pipe connected to the housing portion; and a muffler connected to the second exhaust pipe; the first exhaust pipe including a curved portion, and a straight portion extending linearly from the curved portion, a curved portion, and a straight portion extending linearly from the curved portion, the enlarged portion including an upstream end connected to the straight portion, and a downstream end connected to the housing portion, an upstream end connected to the straight portion, and a downstream end connected to the housing portion, the enlarged portion forming a flow path that enlarges from the upstream end of the enlarged portion to the downstream end of the enlarged portion, and the straight portion being inserted from outside of the enlarged portion into inside of the enlarged portion through the upstream end of the enlarged portion, and extending toward the catalyst; whereby the curved portion has a central axis passing through the center of a flow path in the curved portion, the curved portion includes an inner curved portion positioned more inward than the central axis of the curved portion, having a U-shaped cross sectional face, and extending along the central axis of the curved portion, and an outer curved portion positioned more outward than the central axis of the curved portion, having a U-shaped cross sectional face, and extending along the central axis of the curved portion, characterised in that the straight portion includes an inner straight portion continuing to the inner curved portion, and an outer straight portion continuing to the outer curved portion, and an inner curved portion positioned more inward than the central axis of the curved portion, having a U-shaped cross sectional face, and extending along the central axis of the curved portion, and an outer curved portion positioned more outward than the central axis of the curved portion, having a U-shaped cross sectional face, and extending along the central axis of the curved portion, characterised in that the straight portion includes an inner straight portion continuing to the inner curved portion, and an outer straight portion continuing to the outer curved portion, and a length of the outer straight portion is larger than a length (Gc) of the inner straight portion.