NOVELTY - A reactor to reform fuel comprises a low temperature reaction unit (12), a high temperature reaction unit (11), and a communicating tube (27) between the two reaction units. At least one of the reaction units is equipped with a projecting surface (12A) and a concave surface (12B), each facing to an opposed surface (11A) of other reaction unit. A length (L1) between the concave surface and the opposed surface is longer than a length (L3) between the projecting surface and the opposed surface. The communication tube is located between the concave surface and the opposed surface.    USE - In a power generation apparatus to reform fuel into hydrogen (claimed), and in fuel cell device.    ADVANTAGE - The reactor suppresses heat loss and causes a chemical reaction at a proper temperature. The low temperature and high temperature reaction units can be formed of same material to minimize the strain at connecting portion of the communicating tube caused by heat expansion when the reaction unit is heated. The distance of the communicating tube is lengthened. Thereby, the heat transfer from high temperature reaction unit to low temperature reaction unit can be suppressed, and the distance between the low and high temperature reaction unit is shortened to make the miniaturization possible. The metallic plate is superior in heat conductance to a glass substrate, and the reactor can be worked to be thin. Thereby, the miniaturization can be realized, and the heat capacity becomes small simultaneously. Thus, chemical reaction can be caused at a proper temperature by small heat quantity. Because the incurrent and excurrent canals are provided so that they may touch each other, the fluid heated by the heat of the low temperature portion heats the fluid flowing into the excurrent canal when the fluid flows out through the excurrent canal. Thereby, heat exchange is enabled, and the decrease of the heat quantity for heating the fluid which has been heated in the incurrent canal in advance in the low temperature reaction unit. Consequently, the chemical reaction can be caused at a proper temperature.    DETAILED DESCRIPTION - A reactor to reform fuel comprises a low temperature reaction unit (12), a high temperature reaction unit (11), and a communicating tube (27) between the two reaction units. At least one of the reaction units is equipped with a projecting surface (12A) and a concave surface (12B), each facing to an opposed surface (11A) of other reaction unit. A length (L3) between the concave surface and the opposed surface is longer than a length (L1) between the projecting surface and the opposed surface. The communication tube is located between the concave surface and the opposed surface. At least one of the reaction units is formed of metallic plates. The reactor has incurrent canal in low temperature reaction unit and excurrent canal to touch the incurrent canal. The high temperature reaction unit includes a heater and a reformer and low temperature reaction unit has carbon monoxide remover, vaporizer and heater.    DESCRIPTION OF DRAWING(S) - The figure shows a sectional view of the power generation apparatus.    high temperature reaction unit (11)    opposed surface of high temperature reaction unit (11A)    low temperature reaction unit (12)    projecting surface of low temperature reaction unit (12A)    concave surface of low surface reaction unit (12B)    communicating tubes (27, 28)    length between projecting surface and opposed surface (L1)    length between concave surface and opposed surface. (L3)