The invention relates to an exhaust gas after treatment apparatus having a hollow cylindrical casing shell (2). In the inside an exhaust catalytic converter, preferably adapted as SCR-catalytic converter, as well as an inflow chamber (9) for exhaust gas to be purified are provided. Therein, the inflow chamber (9) is defined by two bottom portions (10, 11) arranged opposite to each other. Further, a shell portion (12) gas tightly encompassing the bottom portions (10, 11) is provided for the inflow chamber (9). The inflow chamber (9) is encompassed by the casing shell (2) and may be manufactured from a material different from a material of the casing shell.