NOVELTY - A carbon-monoxide-conversion apparatus (1) has catalyst layers (3,4) divided into upstream and downstream stages. Catalyst (I) is provided in the upstream and catalyst (II) is provided in the downstream. When the carbon monoxide concentration in reactive gas (G0) supplied to catalyst (I), and reaction temperature are constant, the carbon monoxide conversion rate reduces. The grade of decline in conversion rate with respect to the increase in carbon dioxide levels in gas supplied to catalyst (II) is smaller than that in gas supplied to catalyst (I).    USE - Carbon-monoxide-conversion apparatus is used for reducing carbon monoxide concentration in reformed gas in hydrogen production apparatus (claimed) used in fuel cells.    ADVANTAGE - The carbon-monoxide-conversion apparatus uses a downstream catalyst having a larger tolerance with respect to carbon-dioxide poisoning than upstream catalyst. Hence, influence of catalytic activity reduction is inhibited and carbon monoxide concentration conversion ratio is improved. The platinum-based catalyst exhibits favorable low-temperature activity. The catalysts are economical. The hydrogen production apparatus using the apparatus improves electric power generation efficiency of fuel cells. The apparatus is compact and economical.    DETAILED DESCRIPTION - A carbon-monoxide-conversion apparatus reacts carbon monoxide and water vapor contained in a reactive gas and converts into carbon dioxide and hydrogen. The apparatus is provided with a transformation catalyst layer divided into an upstream and two downstream stages. Catalyst (I) is provided in the upstream and catalyst (II) is provided in the downstream. When the carbon monoxide concentration in the reactive gas supplied to catalyst (I), and reaction temperature are constant, the carbon monoxide conversion rate reduces such that the carbon dioxide levels in the reactive gas are high. The grade of decline in carbon monoxide conversion rate with respect to the increase in carbon dioxide levels in the reactive gas supplied to catalyst (II) is smaller than the grade of decline in carbon monoxide conversion rate with respect to the increase in carbon dioxide levels in the reactive gas supplied to catalyst (I). INDEPENDENT CLAIMS are included for the following:    (1) carbon-monoxide conversion method; and    (2) hydrogen production apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the schematic structure of the carbon-monoxide-conversion apparatus.    Carbon-monoxide-conversion apparatus (1)    Reaction tube (2)    Catalyst layers (3,4)    Inlet port (5)    Reactive gas (G0)