NOVELTY - The reformer comprises a heating unit (20) comprising a heating unit chamber extending in a first direction and a first combustor (21) arranged to receive and oxidize heating unit fuel and anode off gas, and a reforming unit (10, 11, 12) including a first space formed around the heating unit chamber, a second space formed around the first space and a flow path portion arranged to connect the first space and the second space at an end region of the first and second spaces. The heating unit further comprises a second combustor (22) arranged to receive and oxidize heating unit fuel.    USE - The reformer is useful for fuel cell such as polyelectrolyte fuel cell, a solid oxide fuel cell and a molten carbonate fuel cell.    ADVANTAGE - The reformer has high durability and reliability, improved thermal and combustion efficiency and long-time lifespan, operates the fuel cell for long time in an eco-friendly manner without deterioration or damage, prevents breakage and damage caused due to thermal expansion, and minimizes the thermal shock of the cell caused by repetition of thermal expansion and shrinkage through alleviation or distribution of the hot spots.    DETAILED DESCRIPTION - The reformer comprises a heating unit (20) comprising a heating unit chamber extending in a first direction and a first combustor (21) arranged to receive and oxidize heating unit fuel and anode off gas, and a reforming unit (10, 11, 12) including a first space formed around the heating unit chamber, a second space formed around the first space and a flow path portion arranged to connect the first space and the second space at an end region of the first and second spaces. The heating unit further comprises a second combustor (22) arranged to receive and oxidize heating unit fuel. The reforming unit comprises a deformable region located adjacent to the first combustor, is arranged to reform fuel by a steam reforming reaction in the first space, and is arranged to perform a water gas shift reaction in the second space. The deformable region is arranged to deform in a predetermined manner as result from heat from the first combustor, comprises a curved portion optionally with a U-shaped cross section, and is partially located on an inner wall of the heating unit chamber. The flow path portion comprises a first surface and a second surface that extend partially in a second direction that intersects with the first direction, and is located adjacent to the first combustor. The first surface and/or the second surface of the flow path portion comprise a curved portion that forms part of the deformable region. The curved portion of the first and second surfaces of the flow path portion forms a concentric circle around an axis that extends in the first direction, and includes a convex portion and a U-shaped cross section. The first combustor comprises a nozzle arranged to supply the anode off gas to the heating unit chamber in a direction that intersects with the first direction optionally in a direction perpendicular to the first direction, and is located at a first end of the heating unit chamber. The second combustor is located at an opposite end of the heating unit chamber.    DESCRIPTION OF DRAWING(S) - The diagram shows a longitudinal cross-sectional view of a reformer.    Reforming units (10, 11, 12)    Heating unit (20)    First combustor (21)    Second combustor. (22)