A gas duct having a honeycomb structure includes: a metal case, a honeycomb structure accommodated in the metal case, a holding member placed between the outer surface of the honeycomb structure and the inner surface of the metal case, and a cone fitted to the inner surface of the metal case at one or both openings of the metal case, wherein the circumference of one or both end faces of the honeycomb structure is allowed to abut on the cone. The gas duct having a honeycomb structure can effectively utilize the whole volume of the honeycomb structure, is low in pressure loss, and inexpensive.