A muffler unit comprising a muffler to which exhaust gas is supplied individually from a first exhaust pipe and a second exhaust pipe, the muffler unit further comprising: an inner cylinder connected with the first exhaust pipe; a first outlet pipe that is connected with a downstream end of the inner cylinder and through which the exhaust gas is discharged; an outer cylinder that houses thereinside the inner cylinder and defines, between the outer cylinder and an outer surface of the inner cylinder, an expansion chamber communicating with the second exhaust pipe; a second outlet pipe that houses thereinside the first outlet pipe and defines, between the second outlet pipe and an outer surface of the first outlet pipe, a path connected with the expansion chamber to thereby be used for discharging the exhaust gas; and a partition wall; wherein the outer cylinder includes: an outer cylinder main body that supports the inner cylinder; and a tail end body that is coupled with a downstream end of the outer cylinder main body and supports the first outlet pipe and the second outlet pipe, the muffler unit being characterized in that the partition wall includes a reduced-diameter portion and an enlarged-diameter portion, the reduced-diameter portion has an upstream end joined with an inner surface of the tail end body on an upstream end of the tail end body, the reduced-diameter portion extending, while reducing a diameter thereof, from the upstream end toward a downstream end of the second outlet pipe and being joined with an outer peripheral surface of the second outlet pipe at a downstream end whereas the enlarged-diameter portion is continuous from the downstream end of the reduced-diameter portion and extends, while increasing the diameter thereof, toward a downstream end of the tail end body, the enlarged-diameter portion having a downstream end joined with the inner surface of the tail end body at the downstream end of the tail end body. an inner cylinder connected with the first exhaust pipe; a first outlet pipe that is connected with a downstream end of the inner cylinder and through which the exhaust gas is discharged; an outer cylinder that houses thereinside the inner cylinder and defines, between the outer cylinder and an outer surface of the inner cylinder, an expansion chamber communicating with the second exhaust pipe; a second outlet pipe that houses thereinside the first outlet pipe and defines, between the second outlet pipe and an outer surface of the first outlet pipe, a path connected with the expansion chamber to thereby be used for discharging the exhaust gas; and a partition wall; wherein the outer cylinder includes: an outer cylinder main body that supports the inner cylinder; and a tail end body that is coupled with a downstream end of the outer cylinder main body and supports the first outlet pipe and the second outlet pipe, the muffler unit being characterized in that the partition wall includes a reduced-diameter portion and an enlarged-diameter portion, the reduced-diameter portion has an upstream end joined with an inner surface of the tail end body on an upstream end of the tail end body, the reduced-diameter portion extending, while reducing a diameter thereof, from the upstream end toward a downstream end of the second outlet pipe and being joined with an outer peripheral surface of the second outlet pipe at a downstream end whereas the enlarged-diameter portion is continuous from the downstream end of the reduced-diameter portion and extends, while increasing the diameter thereof, toward a downstream end of the tail end body, the enlarged-diameter portion having a downstream end joined with the inner surface of the tail end body at the downstream end of the tail end body. an outer cylinder main body that supports the inner cylinder; and a tail end body that is coupled with a downstream end of the outer cylinder main body and supports the first outlet pipe and the second outlet pipe, the muffler unit being characterized in that the partition wall includes a reduced-diameter portion and an enlarged-diameter portion, the reduced-diameter portion has an upstream end joined with an inner surface of the tail end body on an upstream end of the tail end body, the reduced-diameter portion extending, while reducing a diameter thereof, from the upstream end toward a downstream end of the second outlet pipe and being joined with an outer peripheral surface of the second outlet pipe at a downstream end whereas the enlarged-diameter portion is continuous from the downstream end of the reduced-diameter portion and extends, while increasing the diameter thereof, toward a downstream end of the tail end body, the enlarged-diameter portion having a downstream end joined with the inner surface of the tail end body at the downstream end of the tail end body.