NOVELTY - A shift reactor (1) consists of reaction portion (3) provided with shift-reaction catalyst layer (2), a reformed-gas inlet tube path (7) having reformed-gas inlet (7a), reaction portion (5) provided with shift-reaction catalyst layer (4), and a communication portion (6). A coolant flow through tube path (8) penetrates layer (2). Reformed gas introduced into layer (2) circulates in head-flow direction in inlet tube path. Reformed gas introduced into portion (5) from reaction (3) circulates through layers (2,4).    USE - Shift reactor is used in fuel cell systems (claimed).    ADVANTAGE - The shift reactor has high utilization efficiency and reaction efficiency with respect to catalyst, and excellent economical efficiency. The shift reactor provides fuel cell systems having excellent electric-power generation efficiency. The usage amount of catalyst is reduced. The shift reactor can be manufactured easily.    DETAILED DESCRIPTION - A shift reactor carries out shift reaction of carbon monoxide and steam in reformed gas obtained by modification reaction of a hydrocarbon raw material and water. The shift reactor consists of reaction portion (3) provided internally with shift-reaction catalyst layer (2), a reformed-gas inlet tube path where front end is inserted inside layer (2) and reformed-gas inlet is provided at the front end, reaction portion (5) surrounding portion (3) and provided internally with shift-reaction catalyst layer (4), and a communication portion which communicates between the reaction portions. A coolant flow through tube path penetrates inside layer (2). The reformed gas introduced into layer (2) from the inlet circulates in layer (2) in head-flow direction in inlet tube path. Reformed gas introduced into reaction portion (5) through the communication portion from reaction portion (3) circulates through layers (2,4) through a gas reaction circulation path of folded structure in head-flow direction.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-sectional view of the shift reactor. (Drawing includes non-English language text)    Shift reactor (1)    Shift-reaction catalyst layers (2,4)    Reaction portions (3,5)    Communication portion (6)    Reformed-gas inlet tube path (7)    Reformed-gas inlet (7a)    Coolant flow through tube path (8)