NOVELTY - A hydrogen production system (20) comprises a fuel supply unit (2) which supplies hydrocarbon fuel, a desulfurization unit (3) having a desulfurization catalyst for desulfurizing hydrocarbon fuel, a hydrogen producing unit (4) for generating hydrogen from hydrocarbon fuel, and a deionization unit (15). The deionization unit brings the hydrocarbon fuel supplied from the fuel supplying unit or the hydrocarbon fuel passed through the desulfurization unit into contact with a porous ion adsorbent.    USE - Hydrogen production system is used for producing hydrogen in fuel cell system (all claimed).    ADVANTAGE - The deionization unit efficiently removes ion derived from the moisture content in hydrocarbon fuel.    DETAILED DESCRIPTION - A hydrogen production system comprises a fuel supply unit which supplies hydrocarbon fuel containing moisture with electrical conductivity of 0.05-500 mu S/cm, and a sulfur compound to a later stage, a desulfurization unit having a desulfurization catalyst for desulfurizing hydrocarbon fuel, a hydrogen producing unit for generating hydrogen from hydrocarbon fuel, and a deionization unit provided between the fuel supply unit and the desulfurization unit, or between the desulfurization unit and the hydrogen producing unit. The deionization unit brings the hydrocarbon fuel supplied from the fuel supplying unit or the hydrocarbon fuel passed through the desulfurization unit into contact with a porous ion adsorbent. INDEPENDENT CLAIMS are included for the following:    (1) deionization of hydrocarbon fuel, which involves contacting hydrocarbon fuel and porous ion adsorbent; and    (2) production of hydrogen, which involves modifying deionized hydrocarbon fuel and obtaining hydrogen.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of fuel cell system. (Drawing includes non-English language text)    Fuel cell system (1)    Fuel supply unit (2)    Desulfurization unit (3)    Hydrogen producing unit (4)    Deionization unit (15)    Hydrogen production system (20)