NOVELTY - A hydrogen-production system has modifier (1), hydrogen and carbon-dioxide separators (2,3), and re-circulating line. The modifier produces reformed gas containing hydrogen, and hydrogen separator separates gas containing hydrogen. The carbon-dioxide separator has separation membrane/absorber. A hot gas is supplied to carbon-dioxide separator, gas containing carbon dioxide is separated, and remainder gas is re-circulated.    USE - For fixed-type fuel cells used for motor vehicles, and hydrogen stations.    ADVANTAGE - The hydrogen-production system efficiently separates carbon dioxide which is a global warming gas. Starting, stopping, and load tracking are eased. The system is compact.    DETAILED DESCRIPTION - The hydrogen-production system has a modifier (1), a hydrogen separator (2), a carbon-dioxide separator (3), and a re-circulating line. The modifier produces a reformed gas containing hydrogen, and the hydrogen separator separates the gas containing high-purity hydrogen from the reformed gas, using a hydrogen separation membrane. The carbon-dioxide separator has a carbon-dioxide separation membrane or a carbon-dioxide absorber. A non-cooled gas in hot state, which does not permeate the hydrogen separation membrane, is supplied to the carbon-dioxide separator, and a gas containing high-purity carbon dioxide is separated. The re-circulating line makes the modifier re-circulate the remainder gas in a hot state while one portion is purged. The reformed gas is obtained by reacting oxygen and water vapor with raw material containing oxygen-containing hydrocarbon and/or hydrocarbon. Oxygen is obtained by separation from air with a de-oxygenation membrane or is supplied separately. The hydrogen separation membrane is a ceramic film. The carbon-dioxide separation membrane consists of a solid-acid compound, and the carbon-dioxide absorber consists of a solid absorber.    DESCRIPTION OF DRAWING(S) - The figure shows the flow sheet of the hydrocarbon modification hydrogen-production system. (Drawing includes non-English language text).    modifier (1)    hydrogen separator (2)    carbon-dioxide separator (3)    exhaust-heat recovery portion (4)    evaporation/superheater (5)