NOVELTY - A fuel cell system comprises hydrogen-producing material container (3) which accommodates hydrogen-producing material which generates hydrogen by exothermic reaction with water, water supply part which supplies water to component 3, hydrogen-producing amount detection part which detects quantity of hydrogen produced, fuel cell (5) which produces electric power using hydrogen as fuel, replacement operation start tide detection part (7), completion tide detection part of replacement operation (8), and replacement indication part (10).    USE - A fuel cell system for portable power supply.    ADVANTAGE - Danger of hot hydrogen and steam ejecting from a cartridge is avoided, and replacement of hydrogen-producing material container is safe.    DETAILED DESCRIPTION - A fuel cell system comprises hydrogen-producing material container (3) which accommodates hydrogen-producing material which generates hydrogen by exothermic reaction with water, water supply part which supplies water to component 3, hydrogen-producing amount detection part which detects quantity of hydrogen produced, fuel cell (5) which produces electric power using hydrogen as fuel, replacement operation start tide detection part (7) which detects start tide of replacement operation of component 3, completion tide detection part of replacement operation (8) which detects completion tide of replacement operation of component 3, and replacement indication part (10) which indicates execution of replacement of component 3. When component 7 detects replacement operation of component 3 during an electrical power generation of component 5, water supply to component 3 is stopped. When the hydrogen-producing amount is less than a predetermined value, component 10 performs replacement of component 3. When component 8 detects completion of tide of a replacement operation, the start tide of water supply to component 3 is performed.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a fuel cell system. (Drawing includes non-English language text).    Hydrogen-producing material container (3)    Fuel cell (5)    Replacement operation start tide detection part (7)    Completion tide detection part of replacement operation (8)    Replacement indication part (10)