The present invention provides a hydrogen generator for generating hydrogen through a steam-reforming process using hydrocarbons as a raw material and a method of operating the same, and, more particularly, provides a hydrogen generator for generating hydrogen through a steam reforming process, which can be stably operated because water is introduced into the hydrogen generator in the form of single phase vapor, and which can achieve high thermal efficiency using a proper heat exchanging method, and to a method of operating the same. According to the present invention, there is provided a heat exchanger network, in which heat necessary for a reforming reaction are obtained by the heat exchange of high-temperature exhaust gas or reformed gas, and in which, in a water gas converting reaction and a PSA reaction conducted at low temperatures compared to the reforming reaction, heat exchange is performed by low-temperature air or water, and the heat-exchanged air and the residual gas in the PSA reaction are used as a heat supply source for the reforming reactor together with fuel hydrocarbons, thereby minimizing the thermal loss of the hydrogen generator.