A reformer is disclosed for supplying a reformed gas containing hydrogen to a fuel cell. The reformer comprises a heat source; a preheating portion preheated by heat from the heat source; a pipe shaped reforming reaction unit; a carbon monoxide processing unit extending from the reforming reaction unit; and a rugged portion installed on an internal surface of the pipe shaped reforming reaction unit which is heated by the heat source. By using the rugged portion and the extended pipe design, the area heated by the heat source is increased and more heat is recovered, thereby improving thermal efficiency.