A mixing device includes: a cylindrical portion having a blade provided inside thereof for mixing exhaust gas that flows inside of an exhaust pipe of an internal combustion engine, with an additive sprayed into the exhaust pipe by an adding valve; a flange having an insertion hole, into which the cylindrical portion is inserted, and extending outward in a radial direction of the cylindrical portion from an outer peripheral surface of the cylindrical portion; and a plurality of supporting portions that are provided on the flange and that support the cylindrical portion, wherein each of the supporting portions includes an extending piece that extends in a circumferential direction of the cylindrical portion from a base end portion connected to the flange.