NOVELTY - The modification reactor has a source-gas flow path (2), a catalyst supported in inner surface of source-gas flow path, a cylindrical gas-separation membrane (20), a transmissive gas airflow passage (21), and a heater (3). The flow path is penetrated and formed in catalyst-support block (1). The gas-separation membrane is arranged along the flow path, and the airflow passage is connected with the gas-separation membrane.    USE - For producing hydrogen-containing gas using hydrocarbon or oxygen-containing hydrocarbon as raw material, for use in fuel cells for vehicles.    ADVANTAGE - The modification reactor is compact and durable, and efficiently carries out modification reaction accompanying heat-absorption effect. The reactor supplies heat energy and promotes modification reaction. The need for ensuring space for temperature rise is eliminated. Since the catalyst support does not form a catalyst layer, pressurization of source gas is not required. Since the support heat-generates, the obtained reaction temperature is acquired without heating. The catalyst activity and separative power are not reduced. Hydrogen gas is efficiently produced from the source gas.    DETAILED DESCRIPTION - The modification reactor has a source-gas flow path (2) which allows a source gas to pass through, a catalyst supported in the inner surface of the source-gas flow path, a cylindrical gas-separation membrane (20), a transmissive gas airflow passage (21), and a heater (3). The flow path is penetrated and formed in a catalyst-support block (1). The gas-separation membrane is arranged along the flow path. The airflow passage removes transmissive gas to the exterior of the catalyst-support block, and is connected with the gas-separation membrane. The heater heats the catalyst-support block, and is embedded in the block.    DESCRIPTION OF DRAWING(S) - The figure shows the exploded perspective view of the modification reactor.    catalyst-support block (1)    source-gas flow path (2)    heater (3)    gas-separation membrane (20)    gas airflow passage (21)