An exhaust gas treatment device for an exhaust system of an internal combustion engine; the treatment device comprises: a tubular duct , which is delimited by a tubular side wall and has an exhaust gas inlet opening , an exhaust gas outlet opening opposite the inlet opening , and an injection opening , which is obtained through the tubular side wall between the inlet opening and the outlet opening and is designed to receive an injector for a reducing substance; and a mixing body , which is arranged inside the tubular duct and creates a forced path to be followed by the exhaust gases in order to flow from the inlet opening to the outlet opening ; wherein the mixing body comprises a first base wall with a partially circular shape, which faces the outlet opening of the tubular duct and partially engages the cross section of the tubular duct leaving free a passage area; wherein the mixing body comprises a second base wall with a circular shape, which faces the inlet opening of the tubular duct , completely engages the cross section of the tubular duct and has, at the centre, a main hole , which is coaxial to the tubular duct and is a through hole in order to allow for the passage of the exhaust gases; and wherein the mixing body comprises a first deflector wall , which extends from the first base wall towards the second base wall ; the treatment device is characterized in that: the first deflector wall forms an acute angle with the first base wall and with the second base wall ; the first base wall partially engages the cross section of the tubular duct so as to leave free only one single passage area shaped like a half-moon and not containing any obstructions; and the second base wall has, at the centre, a single and one and only main hole . a tubular duct , which is delimited by a tubular side wall and has an exhaust gas inlet opening , an exhaust gas outlet opening opposite the inlet opening , and an injection opening , which is obtained through the tubular side wall between the inlet opening and the outlet opening and is designed to receive an injector for a reducing substance; and a mixing body , which is arranged inside the tubular duct and creates a forced path to be followed by the exhaust gases in order to flow from the inlet opening to the outlet opening ; wherein the mixing body comprises a first base wall with a partially circular shape, which faces the outlet opening of the tubular duct and partially engages the cross section of the tubular duct leaving free a passage area; wherein the mixing body comprises a second base wall with a circular shape, which faces the inlet opening of the tubular duct , completely engages the cross section of the tubular duct and has, at the centre, a main hole , which is coaxial to the tubular duct and is a through hole in order to allow for the passage of the exhaust gases; and wherein the mixing body comprises a first deflector wall , which extends from the first base wall towards the second base wall ; the treatment device is characterized in that: the first deflector wall forms an acute angle with the first base wall and with the second base wall ; the first base wall partially engages the cross section of the tubular duct so as to leave free only one single passage area shaped like a half-moon and not containing any obstructions; and the second base wall has, at the centre, a single and one and only main hole . the first deflector wall forms an acute angle with the first base wall and with the second base wall ; the first base wall partially engages the cross section of the tubular duct so as to leave free only one single passage area shaped like a half-moon and not containing any obstructions; and the second base wall has, at the centre, a single and one and only main hole . <EOS>