To provide an exhaust system component having a small diameter neck part capable of being suitably connected to another component on an end part of a tubular member, and a producing method thereof.

This exhaust system component is integrally provided with a tubular body part 101 having a large diameter, the neck part 103 having the small diameter formed on an eccentric position on the end part with respect to a central axis of the body part, and a reduced diameter part comprising a taper part 102 connecting the neck part and the main body part. A predetermined part along the axial direction in the neck part has one and the same cross section. An outside surface of the predetermined part (a connection part 104) positions on an outside along the radial direction of the body part, and constitutes a connection surface CS for the other component. An exhaust system component such as a dual catalytic converter can be constituted by having a plurality of exhaust system parts (a catalytic converter 100) and by holding and integrating the predetermined part of the neck part so as to abut the respective contacting surfaces.