NOVELTY - A hydrogen generator (1) consists of a reforming zone (16) for containing a reforming catalyst to carry out steam reforming reaction of a hydrogen-producing raw material to generate hydrogen, a preheating zone (14), and a heat source (12). The reforming zone, preheating zone, and heat source are arranged in such that heat generated from the heat source is provided to preheating zone and reforming zone, so that hydrogen-producing raw material is primarily preheated in preheating zone and then steam reforming reaction is carried out in reforming zone.    USE - Hydrogen generator is used in hydrogen generation device (claimed) of fuel cells.    ADVANTAGE - The hydrogen generator provides hydrogen-containing gaseous mixture with low carbon monoxide content. Since the surface area of the reforming reactor is enlarged under certain conditions, and a material with a specific thermal conductivity is used as the material for fabricating the reforming reactor, a reforming reactor having excellent thermal conductivity is obtained. The hydrogen generation device presents a desirable temperature distribution during the reaction and, when being used for the steam reforming reaction, provides a hydrogen-containing gaseous mixture which is of a great commercial value. The thermal efficiency of the device is increased.    DETAILED DESCRIPTION - A hydrogen generator essentially composed of medium (I), consists of a reforming zone for containing a reforming catalyst to carry out steam reforming reaction of a hydrogen-producing raw material to generate hydrogen, a preheating zone, and a heat source. The reforming zone, preheating zone, and heat source are arranged in such a way that the heat generated from the heat source is provided to the preheating zone and the reforming zone, so that the hydrogen-producing raw material is primarily preheated in the preheating zone and then steam reforming reaction is carried out in the reforming zone. The reforming zone and preheating zone are divided with medium (I) by a shortest distance of at least 0.5 mm. Medium (I) has a thermal conductivity of at least 60 W/m.K. An INDEPENDENT CLAIM is included for hydrogen generation device.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the hydrogen generator.    Hydrogen generator (1)    Heat source (12)    Preheating zone (14)    Reforming zone (16)    Reforming zone inlet (161)