NOVELTY - The hydrogen production apparatus (1) has fuel modification portion (3), inlet port side-temperature measurement unit (35), reaction completion portion side-temperature measurement unit (37), temperature-gradient calculating unit (57), and reaction completion position calculating unit.    USE - For producing hydrogen for fuel cell system (claimed).    ADVANTAGE - The hydrogen production apparatus enables simple judgment of deterioration of catalyst, even when temperature peak is not acquired in temperature gradient of flow-through direction of gas of catalyst.    DETAILED DESCRIPTION - The hydrogen production apparatus (1) has fuel modification portion (3), inlet port side-temperature measurement unit (35), reaction completion portion side-temperature measurement unit (37), temperature-gradient calculating unit (57) and reaction completion position calculating unit. The fuel modification portion of internal heating has flow path of gas in which modification catalyst which generates reformed gas containing hydrogen by modifying an original fuel. Unit (35) is provided at inlet port side of the original fuel of the catalyst. Unit (37) is provided at a portion of the modification catalyst corresponding to the position of beginning time of catalyst of reaction completion position of catalytic reaction of the gas which flows into the catalyst. Unit (57) calculates the temperature gradient of flow-through direction of the gas of the catalyst based on the temperature measured by unit (35). The reaction completion position measurement unit calculates the position of reaction completion portion based on over time change of the temperature gradient calculated with unit (57). An INDEPENDENT CLAIM is included for fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows the outline block diagram and operation of the hydrogen production apparatus. (Drawing includes non-English language text)    Hydrogen production apparatus (1)    Fuel modification portion (3)    Inlet port side temperature measuring unit (35)    Reaction completion portion side temperature measuring unit (37)    Temperature gradient calculating unit (57)