A casing tube is provided for a honeycomb element used for exhaust gas treatment. The casing tube is provided with a first area which is located close to a first front face and a third area which is located to close a second front face. The casing tube has a first inner diameter in the first and third area and is provided with a second area that is disposed between the first and the third area. The second area has a second inner diameter that is larger than the first inner diameter. The casing tube is characterized in that the axial length of the first area is less than 5 mm, preferably less than 3 mm. The relatively narrow first area enables the honeycomb to be permanently fixed and ensures that only a small amount of heat is transmitted to the casing tube. As a result, a significant improvement is achieved in the cold start performance of a catalytic converter.