NOVELTY - A hydrogen production apparatus consists of a modifier (1) containing modification catalyst, a selective oxidation device (3), and a controller. The modifier generates a source gas from source gas supply device (4) and a reformed gas containing hydrogen by performing steam-reforming reaction using water vapor obtained by vaporizing water from water supply machine (5). The controller starts the supply of source gas from the source gas supply device and supplies water from the water supply machine, when the hydrogen production apparatus is started.    USE - Hydrogen production apparatus is used for domestic fuel cell system used for generation of electric power, and combustor for turbine and engine.    ADVANTAGE - The hydrogen production apparatus has high catalyst activity, and provides generated gas with reduced carbon monoxide content.    DETAILED DESCRIPTION - A hydrogen production apparatus consists of a modifier containing modification catalyst, a selective oxidation device, and a controller. The modifier generates a source gas from source gas supply device and a reformed gas containing hydrogen by performing steam-reforming reaction using water vapor obtained by vaporizing water from water supply machine. The selective oxidation device reduces carbon monoxide in the gas using selective oxidation catalyst flowing-in from the modifier, and mixed gas formed by mixing air from selective oxidation air supply device (6) by selective oxidation reaction. The controller starts the supply of source gas from the source gas supply device and supplies water from the water supply machine, when the hydrogen production apparatus is started. The flow volume of air from selective oxidation air supply device during the supply of air from selective oxidation air supply device is controlled by stopping a fall or supply of air temporarily from a predetermined value. An INDEPENDENT CLAIM is included for method for starting hydrogen production apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of hydrogen production apparatus.    Modifier (1)    Selective oxidation device (3)    Source gas supply device (4)    Water supply machine (5)    Selective oxidation air supply device (6)