NOVELTY - A vaporizing device (30) vaporizes a portion of either of a hydrocarbon fuel or water, to produce a vapor gas containing vapor and air. A vapor mixing device (50) creates a vapor mixture by spraying a portion of non-vaporized fuel or water, towards the vapor gas. A reforming device (60) reforms the fuel in the vapor mixture to reformate gas containing hydrogen.    USE - For reforming liquid hydrocarbon fuel such as gasoline and gaseous hydrocarbon fuel such as methane or ethane to a reformate gas containing hydrogen.    ADVANTAGE - As the hydrocarbon fuel, water and air are mixed immediately before they are supplied to the reforming device, the prereaction of the fuel-water mixture to be reformed prior to its supply to the reforming device, is prevented reliably and hence desired reforming reaction is achieved in reforming device and reforming efficiency is improved. As the vaporizing device vaporizes a portion of either hydrocarbon fuel or water, the size of the vaporizing device is reduced. As the supply quantity of the fuel or water to be sprayed is easily adjusted, the responsiveness with respect to a change in load is increased.    DETAILED DESCRIPTION - The vapor mixing device has a spray nozzle (52) to spray a portion of either the hydrocarbon fuel or the water, towards the vapor gas. The hydrocarbon fuel is either in liquid state or in gaseous state. A return pipe (48) is attached to the spray nozzle to return a portion of hydrocarbon fuel which is not vaporized, to a fuel tank (42). A vapor gas supply quantity adjusting valve (49) for adjusting the quantity of vapor gas supplied by the vaporizing device to the vapor mixing device, is provided in the return pipe. An air supply quantity adjusting valve (28) adjusts the quantity of the air supplied to the vapor mixing device. The temperature of the vapor mixture supplied to the reforming device is controlled so that the temperature of the vapor mixture is less than the lower limit of the regular operating temperature of the reforming device by a predetermined amount.    An INDEPENDENT CLAIM is also included for fuel reforming method.    DESCRIPTION OF DRAWING(S) - The figure shows an outlline block diagram of hydrocarbon based fuel reforming apparatus.    Air supply quantity adjusting valve (28)    Vaporizing device (30)    Fuel tank (42)    Return pipe (48)    Vapor gas supply quantity adjusting valve (49)    Vapor mixing device (50)    Spray nozzle (52)    Reforming device (60)