NOVELTY - The hydrogen generator (4) has a modifier (5), gas suppliers (I,II) (1,3), a carbon-monoxide removal device (7), and a controller (10). Supplier (I) supplies a gas containing nitrogen compound in modifier. The removal device (7) has an oxidation catalyst having an ammonia-poisoning metal, and removes carbon dioxide in hydrogen-containing gas by oxidation using catalyst and oxidation gas. Controller initiates regeneration of oxidation catalyst when parameter correlating to cumulative supply quantity of ammonia into removal device (7) attains a definite threshold.    USE - In fuel cell systems (claimed) used for electric-power generation.    ADVANTAGE - The hydrogen generator eliminates poisoning of oxidation catalyst, and enables stable supply of high-quality hydrogen-containing gas over a long period of time even with a nitrogen-containing raw material. The fuel cell system using the hydrogen generator enables stable electric-power generation over a long period of time.    DETAILED DESCRIPTION - The hydrogen generator (4) consists of a modifier (5) which produces hydrogen-containing gas by modification reaction of raw material, gas suppliers (I,II) (1,3), a carbon-monoxide removal device (7), and a controller (10). Supplier (I) supplies a gas containing nitrogen compound in the modifier. The carbon-dioxide removal device has an oxidation catalyst containing an ammonia-poisoning metal, and removes carbon dioxide in the hydrogen-containing gas by oxidation reaction using the oxidation catalyst and an oxidation gas. Supplier (II) supplies the oxidation gas to the carbon-monoxide removal device. The controller initiates regeneration of the oxidation catalyst when the parameter correlating to the cumulative supply quantity of ammonia into the carbon-monoxide removal device attains a definite threshold. An INDEPENDENT CLAIM is included for operating method of hydrogen generator and fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the fuel cell system. (Drawing includes non-English language text)    Gas suppliers (1,3)    Hydrogen generator (4)    Modifier (5)    Carbon-monoxide removal device (7)    Controller (10)