NOVELTY - The chamber is surrounded by annular zone (14), in which hydrocarbon and water vapor mixture are introduced into the chamber to circulate. The zone is separated to recover heat lost by the chamber and to preheat the mixture, which circulates in the zone. The zone (14) is surrounded by another zone (16), which circulates the water to vaporize, where the water is in direct contact with walls of the annular zone (16). The circulating water in one of the zone separates the two zones and recovers the heat.    USE - The invention deals with hydrogen generating device for power supply of the fuel cells.    ADVANTAGE - Reduced greenhouse effect and the volume of hydrogen. Minimizes thermal loss and improves the output of the hydrogen generator.    DETAILED DESCRIPTION - The water of the zone (16) is mixed with hydrocarbon before introducing the mixture into another zone (14). The device has two walls, which is contact with water at a temperature lower than the temperature of the reaction chamber. The chamber comprises two combustion parts, where one of the part has an evacuated canal and heat is exchanged between the parts of the chamber nearer to the reaction product. The central zone (40) recovers heat from the part to preheat the circulating reagents or fluids, which are separated by cylindrical and smooth wall. The zone (40) further comprises an agent to overheat the mixture of hydrocarbon gas and water vapor. The device further comprises a fine droplets of liquid hydrocarbon that is mixed with water vapor, heat exchange chamber vaporize the droplets, which is not vaporized by the mixture. A spiral fluid circulation is introduced into the chamber to homogenize the temperature in an annular space arranged to the walls of the space and heat is exchanged in the zone through circulation. The fluids are introduced into the zones and the chamber to a pressure higher than the atmospheric pressure.    DESCRIPTION OF DRAWING(S) - The figure shows a hydrogen generating device:    Reaction chamber; (12)    First annular zone; (14)    Second annular zone; (16)    Combustion chamber; (22)    Central zone. (40)