NOVELTY - A thermometry unit (6) measures the inlet temperature of heating gas. A diagnostic unit (5) diagnoses the deterioration state of a modifier by comparing field heating gas inlet temperature with original heating gas modifier inlet temperature and the amount of deterioration of a modification catalyst.    USE - Fuel battery electrical power generating device.    ADVANTAGE - The deterioration state of the modifier is diagnosed using a diagnostic unit.    DETAILED DESCRIPTION - The fuel battery stack (21) of the fuel battery electrical power generating device has a fuel electrode (18), an electrolyte (19) and an oxidation agent electrode (20). The electrolyte (19) has a catalyst which causes hydrogen gas and water vapor to react for generating hydrogen. Thermometry units (6,32,29) measure the heating gas inlet temperature, ejector inlet temperature and heating gas de-sulfurizing temperature, respectively. A deterioration diagnostic unit (5) diagnoses the deterioration state of a modifier by comparing the temperature of respective thermometry units with a predetermined temperature and the amount of deterioration of the modification catalyst and the methane conversion ratio of the modifier. A modifier thermometry unit (41) detects the temperature of the modifier. A data selection unit selects a predetermined temperature and the amount of deterioration of the modification catalyst of the modifier and the methane conversion ratio of the modifier from respective thermometry units and transmits them to a diagnostic unit (5). The deterioration velocity of the modification catalyst and the reduction velocity of the methane conversion ratio is determined based on the output of the deterioration diagnostic unit. A life span diagnostic unit determines the maximum and minimum values of the amount of deterioration of the catalyst which does not affect power generation of the fuel battery.    INDEPENDENT CLAIMS are also included for the following:    (i) a modifier deterioration diagnostic method; and    (ii) a modifier deterioration diagnostic program.    DESCRIPTION OF DRAWING(S) - The figure shows the fuel battery electrical power generating device.    Deterioration diagnostic unit (5)    Thermometry units (6,29,32)    Fuel electrode (18)    Electrolyte (19)    Oxidizing agent electrode (20)    Fuel battery stack (21)    Modifier thermometry unit (41)