To provide a cylindrical member having a proper reduced body part, and having necking parts with proper rigidity up to reaching the opening end of the cylindrical member by continuing with the body part in a fluid treating system with built-in honeycomb structure.

This honeycomb structure is held in the metallic cylindrical member C1 via a cushioning member. The cylindrical member C1 has the body part 11 formed by diametrally contracting an axial directional prescribed range SA of a part for housing at least the cushioning member and the necking parts 12 and 13 formed by necking an end part up to reaching the opening end by including prescribed ranges 11x and 11y of at least one end side of this body part. Thus, a part corresponding to the prescribed ranges 11x and 11y of the necking parts becomes superposing work parts 12a and 13a.