Abstract: A mixer mixes an exhaust gas and a reducer, the mixer comprising a box closed and sealed to the exclusion of an inlet and an outlet, and in which the exhaust gas flows from the inlet to the outlet. A reducer sprayer is arranged so that the reducer can be sprayed into an exhaust gas flow. The box has a substantially cylindrical shape with a bottom, a cover, an outer wall, and a deflector. The bottom and the cover are substantially flat, superimposed substantially parallel, and form faces of a cylinder. The outer wall joins perimeters of the bottom and the cover and forms a generator of the cylinder. The cover and the outer wall are made together in a single piece. The deflector is an inner wall that joins the bottom and the cover substantially perpendicularly, and the bottom and the deflector are made together in a single piece.