Abstract: A natural gas-using hydrogen generator is provided which is able to produce high purity hydrogen from natural gas and water. The generator comprises a desulfurization reactor, a reformer and a PSA. The natural gas-using hydrogen generator also comprises a first pipe line for connecting the reformer with the desulfurization reactor to provide hot combustion gas from the reformer to the desulfurization reactor. A first valve controls air flow being fed to the reformer, which is provided on a second pipe line for feeding air into the reformer. A heat exchanger is provided for exchanging heat between a third pipe line, whose both ends are connected to the second pipe line with the first valve therebetween. A fourth pipe line leads a hot product gas from the reformer to the CO conversion reactor to cool the product gas. A second valve, provided on the third pipe line, controls air flow being fed to the third pipe line.