A heat exchanger tube is constituted by a flat tube body 14 shaped like a plurality of cylindrical tubes arranged mutually proximally in a plane and connected together at mutually proximate portions of the tubes as communicating portions 13. Cylindrical portions 15 corresponding to the cylindrical tubes of the flat tube body 14 have inner peripheries formed with swirling-flow-forming protrusions 16 directed along spiral trajectories coaxial with central axes 0 of the cylindrical portions 15 so that the respective cylindrical portions 15 may have individual swirling flows of the exhaust gas 10.
