Hydrogen generator and fuel cell system

A hydrogen generator comprises a reformer configured to generate a reformed gas containing at least hydrogen and carbon monoxide; a gas supply portion configured to supply an oxidization gas containing oxygen; a purifier configured to reduce a concentration of carbon monoxide contained in the reformed gas in such a manner that a mixture gas containing the reformed gas and the oxidization gas flows through the purifier such that the mixture gas flows through a catalyst body within the purifier to allow the carbon monoxide and the oxygen contained in the mixture gas to react with each other, wherein the purifier has a first catalyst body that selectively oxidizes the carbon monoxide and a second catalyst body which is lower in CO selectivity and lower in temperature of oxidization reaction than the first catalyst body, and the second catalyst body is disposed in parallel with the first catalyst body and on a downstream side of the first catalyst body or adjacent the first catalyst body in a flow of the mixture gas.