NOVELTY - A hydrogen generator comprises a controller configured to perform control, such that a carbon monoxide reduction heater is caused to operate in a stop operation period in a way that a temperature of a carbon monoxide reducing portion is kept higher than a first predetermined temperature, which fails to allow steam present within the carbon monoxide reducing portion to condense.    USE - The hydrogen generator is used for a fuel cell system (claimed). It is used in fuel supply to polymer electrolyte fuel cell. It is useful in e.g. chemical plants requiring generation of high purity hydrogen.    ADVANTAGE - The hydrogen generator inhibits steam from condensing within the carbon monoxide reducing portion, during the stop operation period. The catalysts used in the carbon monoxide reducing portion can be inhibited from deteriorating, which leads to a prolonged lifetime of the fuel cell system.    DETAILED DESCRIPTION - A hydrogen generator comprises a reformer having a reforming catalyst configured to cause a source material and water to react with each other, to generate a hydrogen-rich reformed gas; a reformer heater configured to heat the reformer; a carbon monoxide reducing portion having a carbon monoxide reducing catalyst configured to reduce carbon monoxide contained in the reformed gas; a carbon monoxide reduction heater configured to heat the carbon monoxide reducing portion, carbon monoxide reducing catalyst, and/or reformed gas passing through the carbon monoxide reducing portion; and a controller configured to perform control, such that the carbon monoxide reduction heater is caused to operate in a stop operation period in a way that a temperature of the carbon monoxide reducing portion is kept higher than a first predetermined temperature, which fails to allow steam present within the carbon monoxide reducing portion to condense.    An INDEPENDENT CLAIM is also included for a fuel cell system comprising a hydrogen generator; and a fuel cell configured to generate electric power using hydrogen generated by the hydrogen generator.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram illustrating the configuration of a hydrogen generator.