A muffler that inhibits reduction in engine performance and reduces exhaust noise volume includes a conduit that passes through an expansion chamber. A partition wall is provided at a central section of the conduit. An upstream section upstream from the partition wall includes an outflow section that has a side surface in which outflow holes are formed that allow exhaust gas to flow out to the expansion chamber. A downstream section downstream from the partition wall has an inflow section that has a side surface formed with inflow holes that allow exhaust gas to flow in from the expansion chamber. An inner diameter of at least one section of the downstream section is smaller than an inner diameter of the outflow section.