A process manufactures an exhaust gas treatment device including a tubular circumferential wall (14) elongated along a longitudinal axis (L) with two axial end areas, an exhaust gas treatment unit (52) arranged in the circumferential wall and a closing element (20, 38) connected to the circumferential wall at the two axial end areas. The exhaust gas treatment unit is held at the circumferential wall by a supporting material layer (54) enclosing the exhaust gas treatment unit and supporting same in relation to the circumferential wall. The process includes arranging a first of the two closing elements at one of the axial end areas of the circumferential wall, pushing the exhaust gas treatment unit enclosed by the supporting material layer into the circumferential wall from the other axial end area (18), and arranging a second of the closing elements at the other axial end area of the circumferential wall.