NOVELTY - A cogeneration system (6) comprises reformer (2) which modifies raw fuel to reformed gas of hydrogen main body according to catalyst, fuel cell (3) where reformed gas and oxidizing agent gas are supplied, hot water storage tank which is connected to fuel cell system, process gas burner (4) which burns reformed gas by time composition of reformed gas is stabilized, heat-exchange system which raises water temperature in hot water storage tank using waste heat of fuel cell and reformer, and modifier burner (9) which maintains catalyst temperature of reformer at predetermined temperature.    USE - A cogeneration system.    ADVANTAGE - The cogeneration system has simple structure, and good energy efficiency.    DETAILED DESCRIPTION - A cogeneration system (6) comprises reformer (2) which modifies raw fuel to reformed gas of hydrogen main body according to catalyst, fuel cell (3) where reformed gas and oxidizing agent gas are supplied, hot water storage tank which is connected to fuel cell system, process gas burner (4) which burns reformed gas by time composition of reformed gas by reformer is stabilized, heat-exchange system which raises water temperature in hot water storage tank using waste heat of fuel cell and reformer, first piping which introduces off gas which finished with fuel electrode of fuel cell unreacted to modifier burner, modifier burner (9) which maintains catalyst temperature of reformer at predetermined temperature, second piping which introduces off gas which finished with fuel electrode of fuel cell unreacted to process gas burner, constant flow rate valve positioned at first piping, and solenoid valve positioned at second piping. The constant flow rate valve introduces off gas required in order that modifier burner may maintain catalyst temperature of reformer at predetermined temperature at the time of driving of fuel cell system to modifier burner with constant flow. The opening control of solenoid valve is carried out so that off gas may be introduced to process gas burner at the time of driving of fuel cell system when domestic load is smaller than predetermined value.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of cogeneration system. (Drawing includes non-English language text).    Reformer (2)    Fuel cell (3)    Process gas burner (4)    Cogeneration system (6)    Modifier burner (9)