A muffler for a motor vehicle which serves as a reactor for oxidizing or re-burning unburned constituents in engine exhaust gases. The muffler comprises a heat insulated inner shell in which a chamber is provided with a silencing unit and is kept at a high temperature sufficient to oxidize the unburned constituents, an outer shell covering the heat insulated inner shell, and means for absorbing the thermal expansion of the inner shell provided in a supporting device of the inner shell. An exhaust pipe extends into the inner shell and has one end provided with a collector. The collector has a guide opening to pass engine exhaust gases into the inner shell while achieving mixing of the exhaust gases in the collector thereby to promote oxidizing reaction of the exhaust gases.
